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Subject: bug#79079: 31.0.50; Piped command output is sometimes lost in Eshell
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Hello,

sometimes when executing commands in Eshell with pipes the output seems
to get lost, for example:

git log | head -n 10 | cat

Instead of the expected ten lines of output, there is no output. This
problem does not happen always, but from time to time, so maybe there is
some timing issue regarding the process handling? In case the issue does
not occur immediately one can try this:

while true { echo "test"; git log | head -n 1 | cat }

For such loops the following error might occur:

Creating pipe: Too many open files
Cannot redirect stderr: Too many open files, /dev/null

I tested this with emacs -Q on Emacs 31 with a relatively recent commit
from a few days ago (1e3d76af5a7a16682e71f9df46f6788c0880cccd).

Can someone confirm these problems? Thanks!

In GNU Emacs 31.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version
 3.24.49, cairo version 1.18.4) of 2025-07-15
Windowing system distributor 'The X.Org Foundation', version 11.0.12101016
System Description: Debian GNU/Linux 13 (trixie)

Configured using:
 'configure --prefix=$HOME/.local/share/emacs
 --without-compress-install --with-tree-sitter --with-native-compilation
 --with-dbus --without-selinux --without-threads --disable-gc-mark-trace
 --without-gsettings --without-gpm --with-cairo --with-cairo-xcb
 --with-xinput2 --with-x-toolkit=gtk3 --without-toolkit-scroll-bars
 'CFLAGS=-O3 -mtune=native -march=native''

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS HARFBUZZ JPEG LIBOTF LIBSYSTEMD
LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP
SOUND SQLITE3 TIFF TREE_SITTER WEBP X11 XDBE XIM XINERAMA XINPUT2 XPM
XRANDR GTK3 ZLIB

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From: help-debbugs@HIDDEN (GNU bug Tracking System)
To: Daniel Mendler <mail@HIDDEN>
Subject: bug#79079: Acknowledgement (31.0.50; Piped command output is
 sometimes lost in Eshell)
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Subject: bug#79079: 31.0.50; Piped command output is sometimes lost in Eshell
Resent-From: Jim Porter <jporterbugs@HIDDEN>
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On 7/23/2025 2:55 AM, Daniel Mendler via Bug reports for GNU Emacs, the 
Swiss army knife of text editors wrote:
> sometimes when executing commands in Eshell with pipes the output seems
> to get lost, for example:
[snip]
> Can someone confirm these problems? Thanks!

I'm not able to reproduce this locally, but if you first run 
"eshell-debug process", that will log a bunch of process-related 
information to the buffer "*eshell last cmd*". That would probably have 
some details that at least show where the I/O went missing.

Message sent to bug-gnu-emacs@HIDDEN:

Subject: bug#79079: 31.0.50; Piped command output is sometimes lost in Eshell
Resent-From: Daniel Mendler <mail@HIDDEN>
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Date: Wed, 23 Jul 2025 18:36:56 +0200
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Jim Porter <jporterbugs@HIDDEN> writes:

> On 7/23/2025 2:55 AM, Daniel Mendler via Bug reports for GNU Emacs, the S=
wiss
> army knife of text editors wrote:
>> sometimes when executing commands in Eshell with pipes the output seems
>> to get lost, for example:
> [snip]
>> Can someone confirm these problems? Thanks!
>
> I'm not able to reproduce this locally, but if you first run "eshell-debug
> process", that will log a bunch of process-related information to the buf=
fer
> "*eshell last cmd*". That would probably have some details that at least =
show
> where the I/O went missing.

Thanks. I tried your suggestion. What stood out is that in the failed
runs, the output from the git process is received, and passed on to the
head process, but no output is received from head. It seems that the
head filter function is never called? This would hint at a lower-level
problem. See the logs of the runs below.

The problem occurs in particular for this command:

git log --color=3Dalways --decorate=3Dyes | head -n 1

While it does not happen (or not so often) for this:

git log --color=3Dalways --decorate=3Dyes | cat | head -n 1

Did you try the loop with the echo and the command? Do you have any
further suggestions on how to debug this? Maybe it has something to do
with my specific build.

Daniel

### Good run
[process] started external process =E2=80=98head=E2=80=99
[process] started external process =E2=80=98git=E2=80=99
[process] received output from process =E2=80=98git=E2=80=99
[process] forwarding output from process =E2=80=98git=E2=80=99
[process] received output from process =E2=80=98head=E2=80=99 <=3D=3D=3D=3D=
=3D received output
[process] received output from process =E2=80=98git=E2=80=99
[process] forwarding output from process =E2=80=98git=E2=80=99
[process] received output from process =E2=80=98git=E2=80=99
[process] forwarding output from process =E2=80=98git=E2=80=99
[process] sentinel for external process =E2=80=98git=E2=80=99: "broken pipe
[process] i/o busy for process =E2=80=98git=E2=80=99
[process] sentinel for external process =E2=80=98git-stderr=E2=80=99: "fini=
shed
[process] finished external process =E2=80=98git-stderr=E2=80=99
[process] sentinel for external process =E2=80=98head=E2=80=99: "finished <=
=3D=3D=3D=3D=3D finished
[process] finished external process =E2=80=98head=E2=80=99
[process] finished external process =E2=80=98git=E2=80=99

### Bad run
[process] started external process =E2=80=98head=E2=80=99
[process] started external process =E2=80=98git=E2=80=99
[process] received output from process =E2=80=98git=E2=80=99
[process] forwarding output from process =E2=80=98git=E2=80=99
[process] received output from process =E2=80=98git=E2=80=99
[process] forwarding output from process =E2=80=98git=E2=80=99
[process] received output from process =E2=80=98git=E2=80=99
[process] forwarding output from process =E2=80=98git=E2=80=99
[process] sentinel for external process =E2=80=98head=E2=80=99: "finished
[process] finished external process =E2=80=98head=E2=80=99 <=3D=3D=3D=3D=3D=
 finished without output
[process] sentinel for external process =E2=80=98git=E2=80=99: "broken pipe
[process] i/o busy for process =E2=80=98git=E2=80=99
[process] sentinel for external process =E2=80=98git-stderr=E2=80=99: "fini=
shed
[process] finished external process =E2=80=98git-stderr=E2=80=99
[process] finished external process =E2=80=98git=E2=80=99

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On 7/23/2025 8:52 AM, Jim Porter wrote:
> I'm not able to reproduce this locally, but if you first run 
> "eshell-debug process", that will log a bunch of process-related 
> information to the buffer "*eshell last cmd*". That would probably have 
> some details that at least show where the I/O went missing.

After a few more tries, I was able to reproduce this very rarely. Does 
the following patch help?

Eli: maybe you can help with some of the details here. In process.c, 
when we write to a process, we handle EPIPE errors by calling 
'deactivate_process'. However, that can lead to us dropping any data 
written *by* that process, since we don't call the process's filter 
function for any remaining output still in our internal buffer.

I think in this case, we'd just want to let the rest of our code handle 
deactivating the process in the usual way. That helps fix this bug, plus 
I think it makes sense in general. If a process closes stdin, I believe 
we'd get the EPIPE error, but that process might want to continue 
working (though it does mean that you could only interact with that 
process via signaling it).

Does that make sense?
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Jim Porter <jporterbugs@HIDDEN> writes:

> On 7/23/2025 8:52 AM, Jim Porter wrote:
>> I'm not able to reproduce this locally, but if you first run "eshell-debug
>> process", that will log a bunch of process-related information to the buffer
>> "*eshell last cmd*". That would probably have some details that at least show
>> where the I/O went missing.
>
> After a few more tries, I was able to reproduce this very rarely. Does the
> following patch help?

Thanks for looking into this so quickly and coming up with a patch! I
can check this earliest end of next week. Recompiling Emacs always takes
me a little bit longer than simply evaluating some Elisp.

> Eli: maybe you can help with some of the details here. In process.c, when we
> write to a process, we handle EPIPE errors by calling 'deactivate_process'.
> However, that can lead to us dropping any data written *by* that process, since
> we don't call the process's filter function for any remaining output still in
> our internal buffer.
>
> I think in this case, we'd just want to let the rest of our code handle
> deactivating the process in the usual way. That helps fix this bug, plus I think
> it makes sense in general. If a process closes stdin, I believe we'd get the
> EPIPE error, but that process might want to continue working (though it does
> mean that you could only interact with that process via signaling it).

Maybe this also needs some test to avoid regressions? Did you find a way
to reliably reproduce the problem?

> Does that make sense?
>
> diff --git a/src/process.c b/src/process.c
> index e61ec425f7e..258bee21b5e 100644
> --- a/src/process.c
> +++ b/src/process.c
> @@ -6876,7 +6876,6 @@ send_process (Lisp_Object proc, const char *buf, ptrdiff_t len,
>                   p->raw_status_new = 0;
>                   pset_status (p, list2 (Qexit, make_fixnum (256)));
>                   p->tick = ++process_tick;
> -                 deactivate_process (proc);
>                   error ("Process %s no longer connected to pipe; closed it",
>                          SDATA (p->name));
>                 }

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Subject: bug#79079: 31.0.50; Piped command output is sometimes lost in Eshell
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> Date: Wed, 23 Jul 2025 11:06:24 -0700
> From: Jim Porter <jporterbugs@HIDDEN>
> 
> On 7/23/2025 8:52 AM, Jim Porter wrote:
> > I'm not able to reproduce this locally, but if you first run 
> > "eshell-debug process", that will log a bunch of process-related 
> > information to the buffer "*eshell last cmd*". That would probably have 
> > some details that at least show where the I/O went missing.
> 
> After a few more tries, I was able to reproduce this very rarely. Does 
> the following patch help?
> 
> Eli: maybe you can help with some of the details here. In process.c, 
> when we write to a process, we handle EPIPE errors by calling 
> 'deactivate_process'. However, that can lead to us dropping any data 
> written *by* that process, since we don't call the process's filter 
> function for any remaining output still in our internal buffer.
> 
> I think in this case, we'd just want to let the rest of our code handle 
> deactivating the process in the usual way. That helps fix this bug, plus 
> I think it makes sense in general. If a process closes stdin, I believe 
> we'd get the EPIPE error, but that process might want to continue 
> working (though it does mean that you could only interact with that 
> process via signaling it).
> 
> Does that make sense?

Sounds a somewhat scary change, since this code was last touched 13
years ago.  Paul, WDYT?

What about calling the filter with the data we still have?  Isn't that
what a Posix shell would do -- flush any buffers?

And why do we get EPIPE in the recipe in this bug, anyway?  I'd like
to understand better at least one situation where we get EPIPE while
the data received before that still matters.

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On 2025-07-23 22:34, Eli Zaretskii wrote:
>> Date: Wed, 23 Jul 2025 11:06:24 -0700
>> From: Jim Porter<jporterbugs@HIDDEN>

>> In process.c,
>> when we write to a process, we handle EPIPE errors by calling
>> 'deactivate_process'. However, that can lead to us dropping any data
>> written*by* that process, since we don't call the process's filter
>> function for any remaining output still in our internal buffer.
>>
>> I think in this case, we'd just want to let the rest of our code handle
>> deactivating the process in the usual way. That helps fix this bug, plus
>> I think it makes sense in general. If a process closes stdin, I believe
>> we'd get the EPIPE error, but that process might want to continue
>> working (though it does mean that you could only interact with that
>> process via signaling it).

Yes, that makes sense.


> What about calling the filter with the data we still have?  Isn't that
> what a Posix shell would do -- flush any buffers?

Not sure what you man by that, but I suspect it wouldn't handle the 
scenario Jim describes, where the other process closes its end of the 
pipe but keeps running. Emacs shouldn't assume that EPIPE means the 
other process has died or has closed any pipes other than the one Emacs 
got EPIPE on.

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> Date: Thu, 24 Jul 2025 06:52:39 -0700
> Cc: mail@HIDDEN, 79079 <at> debbugs.gnu.org,
>  Jim Porter <jporterbugs@HIDDEN>
> From: Paul Eggert <eggert@HIDDEN>
> 
> >> I think in this case, we'd just want to let the rest of our code handle
> >> deactivating the process in the usual way. That helps fix this bug, plus
> >> I think it makes sense in general. If a process closes stdin, I believe
> >> we'd get the EPIPE error, but that process might want to continue
> >> working (though it does mean that you could only interact with that
> >> process via signaling it).
> 
> Yes, that makes sense.
> 
> 
> > What about calling the filter with the data we still have?  Isn't that
> > what a Posix shell would do -- flush any buffers?
> 
> Not sure what you man by that, but I suspect it wouldn't handle the 
> scenario Jim describes, where the other process closes its end of the 
> pipe but keeps running. Emacs shouldn't assume that EPIPE means the 
> other process has died or has closed any pipes other than the one Emacs 
> got EPIPE on.

I guess I'm confused: if the other process closed its end of the pipe,
why do we care about the stuff we have in our buffers that wasn't yet
piped to that other process?

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On 7/23/2025 10:34 PM, Eli Zaretskii wrote:
> What about calling the filter with the data we still have?  Isn't that
> what a Posix shell would do -- flush any buffers?
> 
> And why do we get EPIPE in the recipe in this bug, anyway?  I'd like
> to understand better at least one situation where we get EPIPE while
> the data received before that still matters.

In the recipe, we pipe "git log" to "head -n 10", where Eshell handles 
the actual piping. (I don't think Eshell's presence here is actually 
necessary to see this bug, though it's probably the simplest way to 
reproduce it in Emacs.)

"git log" produces a bunch of output that gets sent to Eshell's process 
filter. That filter eventually calls 'process-send-string' on the "head" 
process, forwarding all of git's stdout to head's stdin. Immediately 
after that, head sees that it's already gotten 10 lines of input, so it 
exits. "git log" doesn't know that yet, so it sends another batch of 
text to Eshell's process filter, which again gets passed to 
'process-send-string'. This time around though, the pipe to head's stdin 
has been closed, so we get an EPIPE. Currently, Emacs deactivates the 
head process, but because of how the timings worked out, all this 
happens *before* Eshell's process filter for head ever runs. As a 
result, Emacs never see the output that head produced.

Thinking about this more, my previous comment that the output from the 
head process is in an internal buffer in Emacs probably isn't accurate. 
That *could* happen, but in some cases, we may not have called 'read' 
yet, so the data could still be in the OS's buffer. (I'd have to write 
some test cases to be 100% sure of all this, but it follows from the 
manpages.)

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On 2025-07-24 07:56, Eli Zaretskii wrote:
>> Not sure what you man by that, but I suspect it wouldn't handle the
>> scenario Jim describes, where the other process closes its end of the
>> pipe but keeps running. Emacs shouldn't assume that EPIPE means the
>> other process has died or has closed any pipes other than the one Emacs
>> got EPIPE on.
> I guess I'm confused: if the other process closed its end of the pipe,
> why do we care about the stuff we have in our buffers that wasn't yet
> piped to that other process?

I could well be the confused one, but the scenario I was concerned about 
is when we are running the process for some reason other than the fact 
that we are piping to it. For example, we have another pipe to the same 
process. In that case we shouldn't deactivate the entire process (i.e., 
turn off all output and input for it) merely because one of its pipes 
closed.

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On 7/24/2025 7:56 AM, Eli Zaretskii wrote:
>> From: Paul Eggert <eggert@HIDDEN>
>>
>> Not sure what you man by that, but I suspect it wouldn't handle the
>> scenario Jim describes, where the other process closes its end of the
>> pipe but keeps running. Emacs shouldn't assume that EPIPE means the
>> other process has died or has closed any pipes other than the one Emacs
>> got EPIPE on.
> 
> I guess I'm confused: if the other process closed its end of the pipe,
> why do we care about the stuff we have in our buffers that wasn't yet
> piped to that other process?

We could have a situation where a process closes stdin, but we still 
haven't read everything (or flushed our buffers) for that process's stdout.

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On 7/24/2025 6:52 AM, Paul Eggert wrote:
> On 2025-07-23 22:34, Eli Zaretskii wrote:
>>> Date: Wed, 23 Jul 2025 11:06:24 -0700
>>> From: Jim Porter<jporterbugs@HIDDEN>
> 
>>> In process.c,
>>> when we write to a process, we handle EPIPE errors by calling
>>> 'deactivate_process'. However, that can lead to us dropping any data
>>> written*by* that process, since we don't call the process's filter
>>> function for any remaining output still in our internal buffer.
>>>
>>> I think in this case, we'd just want to let the rest of our code handle
>>> deactivating the process in the usual way. That helps fix this bug, plus
>>> I think it makes sense in general. If a process closes stdin, I believe
>>> we'd get the EPIPE error, but that process might want to continue
>>> working (though it does mean that you could only interact with that
>>> process via signaling it).
> 
> Yes, that makes sense.

Thanks for checking. Then I think the attached diff would be close to 
what we want, with a few caveats. (The original diff was just the 
smallest change that seemed to fix the problem so that Daniel could try 
it out.)

This diff changes things so that when we get an EPIPE, we close our end 
of the pipe (the 'WRITE_TO_SUBPROCESS' fd). I don't know if we need to 
do anything special for PTYs or sockets though.

On the Eshell side then, we previously detected EPIPE by checking 
whether the process had been deactivated. Since that doesn't happen 
anymore, I changed it so that we always treat an error from 
'process-send-string' as though the pipe broke. In practice, this just 
means that Eshell will think that all future calls to 
'process-send-string' would fail too (and so it will send a SIGPIPE 
signal to the sending process; "git" in the original test case). I'm not 
totally sure that's right though. Could there be a time when we get an 
error from 'process-send-string', but we can try calling it again later?
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On 2025-07-24 10:05, Jim Porter wrote:
> Could there be a time when we get an error from 'process-send-string', 
> but we can try calling it again later?

Not if it's an EPIPE failure. EPIPE means the connection is permanently 
down.

I don't offhand know of any failure for writing to a pipe where retrying 
would make sense, other than EAGAIN and EWOULDBLOCK which the code is 
already dealing with.

Can process-send-string write to other file types? If so, there are 
other possibilities where retrying might work, e.g., ENOSPC for write to 
a regular file, ENETDOWN for sendto to a network socket.

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On 7/24/2025 11:50 AM, Paul Eggert wrote:
> On 2025-07-24 10:05, Jim Porter wrote:
>> Could there be a time when we get an error from 'process-send-string', 
>> but we can try calling it again later?
> 
> Not if it's an EPIPE failure. EPIPE means the connection is permanently 
> down.

Right. I was just wondering if there might be some use in signaling 
different types of errors in send_process; one for transient errors (if 
there are any we care about), and one for permanent errors (which look 
to be most/all of them).

> I don't offhand know of any failure for writing to a pipe where retrying 
> would make sense, other than EAGAIN and EWOULDBLOCK which the code is 
> already dealing with.
> 
> Can process-send-string write to other file types? If so, there are 
> other possibilities where retrying might work, e.g., ENOSPC for write to 
> a regular file, ENETDOWN for sendto to a network socket.

For Eshell, we only support writing to regular processes, though I 
suppose with enough effort you could probably construct an Eshell 
command that writes to a socket. Of course, for something like ENETDOWN, 
Eshell would probably need some special handling to retry sending the 
same data again, instead of just skipping over it.

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> Date: Thu, 24 Jul 2025 10:05:50 -0700
> Cc: mail@HIDDEN, 79079 <at> debbugs.gnu.org
> From: Jim Porter <jporterbugs@HIDDEN>
> 
> --- a/src/process.c
> +++ b/src/process.c
> @@ -6873,10 +6873,13 @@ send_process (Lisp_Object proc, const char *buf, ptrdiff_t len,
>  		}
>  	      else if (errno == EPIPE)
>  		{
> -		  p->raw_status_new = 0;
> -		  pset_status (p, list2 (Qexit, make_fixnum (256)));
> -		  p->tick = ++process_tick;
> -		  deactivate_process (proc);
> +		  /* FIXME: Handle PTY too?  What about sockets?  And
> +		     remove this debug statement.  */
> +		  AUTO_STRING (format, "[debug] EPIPE: %S");
> +		  CALLN (Fmessage, format, p->name);
> +
> +		  close_process_fd(&p->open_fd[WRITE_TO_SUBPROCESS]);
> +		  p->outfd = -1;
>  		  error ("Process %s no longer connected to pipe; closed it",
>  			 SDATA (p->name));
>  		}

Why do we need to emit a message about EPIPE, when we will immediately
signal an error with the same information?  The message will not be
seen by users except if they look in *Messages*.

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On 7/24/2025 11:42 PM, Eli Zaretskii wrote:
> Why do we need to emit a message about EPIPE, when we will immediately
> signal an error with the same information?  The message will not be
> seen by users except if they look in *Messages*.

It's just a debug statement for development/testing so that it's clear 
when Emacs runs this block. Since this is a timing-related issue, it's 
hard to guarantee that we hit this condition. While I'm working on this, 
I can then check *Messages* to be sure my new code ran and did the right 
thing.

I'll remove the Fmessage call before merging.

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> Date: Thu, 24 Jul 2025 08:52:16 -0700
> Cc: mail@HIDDEN, 79079 <at> debbugs.gnu.org
> From: Jim Porter <jporterbugs@HIDDEN>
> 
> On 7/23/2025 10:34 PM, Eli Zaretskii wrote:
> > What about calling the filter with the data we still have?  Isn't that
> > what a Posix shell would do -- flush any buffers?
> > 
> > And why do we get EPIPE in the recipe in this bug, anyway?  I'd like
> > to understand better at least one situation where we get EPIPE while
> > the data received before that still matters.
> 
> In the recipe, we pipe "git log" to "head -n 10", where Eshell handles 
> the actual piping. (I don't think Eshell's presence here is actually 
> necessary to see this bug, though it's probably the simplest way to 
> reproduce it in Emacs.)
> 
> "git log" produces a bunch of output that gets sent to Eshell's process 
> filter. That filter eventually calls 'process-send-string' on the "head" 
> process, forwarding all of git's stdout to head's stdin. Immediately 
> after that, head sees that it's already gotten 10 lines of input, so it 
> exits. "git log" doesn't know that yet, so it sends another batch of 
> text to Eshell's process filter, which again gets passed to 
> 'process-send-string'. This time around though, the pipe to head's stdin 
> has been closed, so we get an EPIPE. Currently, Emacs deactivates the 
> head process, but because of how the timings worked out, all this 
> happens *before* Eshell's process filter for head ever runs. As a 
> result, Emacs never see the output that head produced.

So calling the filter of the "head"s process when we get EPIPE should
fix that, no?

Last modified: Sat, 26 Jul 2025 10:15:01 UTC

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