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lib/create.c: Capture and raise qemu-img stderr #200

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Merged
merged 2 commits into from
May 21, 2025
Merged

lib/create.c: Capture and raise qemu-img stderr #200

merged 2 commits into from
May 21, 2025

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crobinso
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https://issues.redhat.com/browse/RHEL-92239

After this, output looks like

$ ./run guestfish --ro --format=qcow222 -a test.img
libguestfs: error: qemu-img: qemu-img: .../overlay1.qcow2: Unknown driver 'qcow222'
Could not open backing image.
: qemu-img exited with error status 1.
To see full error messages you may need to enable debugging.
...

This is ugly as is, but I'm looking for advice before I go off into the weeds.

Before this commit:

  • stdout is thrown away
  • stderr always ends up in guestfs trace log
  • error message just mentions exit status and qemu-img filename

After this commit

  • combines stderr and stdout and crams it into guestfs_int_external_command_failed when command fails.
  • debug() prints it all on command success

IMO ideal would be:

  • stderr, but not stdout, reported on error
  • guestfs_int_external_command_failed boiler plate is still printed, but formatted nicely.
  • stderr still shows up in some debug output, when command succeeds.

But decoupling the stdout piece may need lib/command.c additions, and formatting the error may need lib/errors.c additions.

@rwmjones suggestions?

@rwmjones
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Seems fine actually. Would it be possible to convert \n chars to a space? (Might consider adding that feature to lib/command.c).

Also what's the output if the --format parameter is a valid format name, but not the correct format for the backing file (ie the original bug report)?

@crobinso crobinso force-pushed the qemu-img-error-output branch from 5701ff6 to 2792029 Compare May 20, 2025 18:10
@crobinso
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Here's output with newlines replaced. I adjust patches to do the newline replacement unconditionally in guestfs_int_external_command_failed which seems reasonable.

$ ./run guestfish --ro --format=qcow2 -a test.img
 libguestfs: error: qemu-img: qemu-img: /home/crobinso/src/libguestfs/tmp/libguestfsFlxnb0/overlay1.qcow2: Image is not in qcow2 format Could not open backing image. : qemu-img exited with error status 1.
 To see full error messages you may need to enable debugging.
...

@crobinso crobinso changed the title WIP: lib/create.c: Capture and raise qemu-img stderr lib/create.c: Capture and raise qemu-img stderr May 20, 2025
rwmjones
rwmjones reviewed May 20, 2025
View reviewed changes
crobinso added 2 commits May 21, 2025 11:38
@crobinso
lib: flatten extra output when external command fails
a7d0a38 
Otherwise string output looks quite awkward

Signed-off-by: Cole Robinson <[email protected]>
@crobinso
lib/create.c: Capture and raise qemu-img stderr
5e01824 
https://issues.redhat.com/browse/RHEL-92239

After this, output looks like

   $ ./run guestfish --ro --format=qcow2 -a test.img
   libguestfs: error: qemu-img: qemu-img: /home/crobinso/src/libguestfs/tmp/libguestfsFlxnb0/overlay1.qcow2: Image is not in qcow2 format Could not open backing image. : qemu-img exited with error status 1.
   To see full error messages you may need to enable debugging.
   ...

Signed-off-by: Cole Robinson <[email protected]>
@crobinso crobinso force-pushed the qemu-img-error-output branch from 2792029 to 5e01824 Compare May 21, 2025 15:39
@rwmjones rwmjones merged commit 606aa1d into libguestfs:master May 21, 2025
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@crobinso @rwmjones