fix(errors): reformat plugin build error output

[cmatsuoka]

In case of Python plugin build errors, print error messages to stderr
instead of stdout so they can be captured and properly displayed to the
user. Don't start printing from the last script execution trace line because
messages printed to stderr appear before the traced exit line.

Fixes #1000

Signed-off-by: Claudio Matsuoka [email protected]

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[cmatsuoka]

This is a very interesting error because it seems that, despite being reported as happening randomly, it should have never worked in the first place: only stderr was captured and the string being tested by Rockcraft was sent to stdout. Now we're capturing the combined output, and accounting for the race of stdout text appearing before or after the +exit 1 trace message sent to stderr.

[cmatsuoka]

It seems that related Rockcraft issues are gone after preventing the Python interpreter search to return an error and stop the build process. See canonical/rockcraft#809

[cmatsuoka]

Note that capturing only stderr can sometimes show us error messages with the echo command trace and the corresponding output. This is still better than displaying output starting at the last trace line -- in this case it would show only the exit 1 line.

Failed to run the build script for part 'my-part'.
Detailed information: 
:: + echo 'No suitable Python interpreter found, giving up.'
:: No suitable Python interpreter found, giving up.
:: + exit 1
Recommended resolution: Check the build output and verify the project can work with the 'python' plugin.

[cmatsuoka]

Also note that capturing the last three lines will capture only the last error message, along with its echo command and exit 1. We can increase the amount of captured lines, at the risk of increasing noise if e.g. a compiler command line is captured.

[cmatsuoka]

As a reference, this is how the error message usually appears if using combined output:

Failed to run the build script for part 'my-part'.
Detailed information: 
:: + exit 1
:: Python interpreter not found in payload.
:: No suitable Python interpreter found, giving up.
Recommended resolution: Check the build output and verify the project can work with the 'python' plugin.

[cmatsuoka]

Not a huge difference in this case, but here's the output including the last three traced lines:

Failed to run the build script for part 'my-part'.
Detailed information: 
:: + '[' -z '' ']'
:: + echo 'No suitable Python interpreter found, giving up.'
:: No suitable Python interpreter found, giving up.
:: + exit 1
Recommended resolution: Check the build output and verify the project can work with the 'python' plugin.

[bepri]

Last 3 commands looks good, I'm happy with this! Thanks.

[tigarmo]

thanks!