progress: disable truncating by default when using --progress=plain

[thaJeztah]

fixes #1412

Currently, docker build --progress=plain truncates output, which makes it
difficult to debug problems during build (the step that failed may not be the
cause of the failure).

For example, in the following build, the output of the RUN echo ... is truncated:

DOCKER_BUILDKIT=1 docker build --no-cache --progress=plain -t foo -<<EOF
FROM busybox
RUN echo "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Maecenas ut lorem nec leo euismod vestibulum. Donec tortor nisl, iaculis at vehicula vel, interdum eu orci. Integer velit lacus, congue id magna eu, mollis accumsan augue. Aliquam non venenatis risus, eu posuere libero. Vestibulum ante ipsum primis in faucibus orci luctus." > /somewhere
RUN echo "something went wrong"; exit 1
EOF

#5 [2/3] RUN echo "Lorem ipsum dolor sit amet, consectetur adipiscing elit....
#5 DONE 0.2s

#6 [3/3] RUN echo "something went wrong"; exit 1
#6 0.211 something went wrong
#6 ERROR: executor failed running [/bin/sh -c echo "something went wrong"; exit 1]: runc did not terminate sucessfully
------
 > [3/3] RUN echo "something went wrong"; exit 1:
------
failed to solve with frontend dockerfile.v0: failed to build LLB: executor failed running [/bin/sh -c echo "something went wrong"; exit 1]: runc did not terminate sucessfully

While there is an existing PROGRESS_NO_TRUNC environment variable, I think that
this should be the default if the user opted to use --progress=plain (or in
situations where no TTY is attached, which could be in CI).

This patch changes the default to disable truncating in those situations, but
allowing users to opt-out by setting PROGRESS_NO_TRUNC=0

With this change the same build looks like this:

#5 [2/3] RUN echo "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Maecenas ut lorem nec leo euismod vestibulum. Donec tortor nisl, iaculis at vehicula vel, interdum eu orci. Integer velit lacus, congue id magna eu, mollis accumsan augue. Aliquam non venenatis risus, eu posuere libero. Vestibulum ante ipsum primis in faucibus orci luctus." > /somewhere
#5 sha256:b2f0f47d63911ee55e7cf17c81007310e28190b5be84aa1a7869ba90786d5cee
#5 DONE 0.2s

#6 [3/3] RUN echo "something went wrong"; exit 1
#6 sha256:c037b34bb998ae7d30572b489286da14df87e1478adf6d0c5c71c79b84b11bcc
#6 0.293 something went wrong
#6 ERROR: executor failed running [/bin/sh -c echo "something went wrong"; exit 1]: runc did not terminate sucessfully
------
 > [3/3] RUN echo "something went wrong"; exit 1:
------
failed to solve with frontend dockerfile.v0: failed to build LLB: executor failed running [/bin/sh -c echo "something went wrong"; exit 1]: runc did not terminate sucessfully

[tonistiigi]

The idea behind the truncation even on plain mode was that the prefix numbers stay aligned. Do you have screenshots on how this looks compared to old. There is also the ... case when parallel blocks switch. Maybe that case should remain truncated as you already have the full line once or maybe that is even more confusing.

[thaJeztah]

Here's two screenshots (I used PROGRESS_NO_TRUNC=1 to mimic the output as I didn't have a docker build with this change, but the output should be the same;

Before:

After:

The idea behind the truncation even on plain mode was that the prefix numbers stay aligned

Do you think we should "hard" wrap the lines and align with the prefix number?

[tonistiigi]

Do you think we should "hard" wrap the lines and align with the prefix number?

This isn't possible without knowing the actual screen width. And if we are on tty then this view isn't shown anyway by default.