git/t/t0061-run-command.sh
Stefan Beller c553c72eed run-command: add an asynchronous parallel child processor
This allows to run external commands in parallel with ordered output
on stderr.

If we run external commands in parallel we cannot pipe the output directly
to the our stdout/err as it would mix up. So each process's output will
flow through a pipe, which we buffer. One subprocess can be directly
piped to out stdout/err for a low latency feedback to the user.

Example:
Let's assume we have 5 submodules A,B,C,D,E and each fetch takes a
different amount of time as the different submodules vary in size, then
the output of fetches in sequential order might look like this:

 time -->
 output: |---A---| |-B-| |-------C-------| |-D-| |-E-|

When we schedule these submodules into maximal two parallel processes,
a schedule and sample output over time may look like this:

process 1: |---A---| |-D-| |-E-|

process 2: |-B-| |-------C-------|

output:    |---A---|B|---C-------|DE

So A will be perceived as it would run normally in the single child
version. As B has finished by the time A is done, we can dump its whole
progress buffer on stderr, such that it looks like it finished in no
time. Once that is done, C is determined to be the visible child and
its progress will be reported in real time.

So this way of output is really good for human consumption, as it only
changes the timing, not the actual output.

For machine consumption the output needs to be prepared in the tasks,
by either having a prefix per line or per block to indicate whose tasks
output is displayed, because the output order may not follow the
original sequential ordering:

 |----A----| |--B--| |-C-|

will be scheduled to be all parallel:

process 1: |----A----|
process 2: |--B--|
process 3: |-C-|
output:    |----A----|CB

This happens because C finished before B did, so it will be queued for
output before B.

To detect when a child has finished executing, we check interleaved
with other actions (such as checking the liveliness of children or
starting new processes) whether the stderr pipe still exists. Once a
child closed its stderr stream, we assume it is terminating very soon,
and use `finish_command()` from the single external process execution
interface to collect the exit status.

By maintaining the strong assumption of stderr being open until the
very end of a child process, we can avoid other hassle such as an
implementation using `waitpid(-1)`, which is not implemented in Windows.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2015-12-16 12:06:08 -08:00

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#!/bin/sh
#
# Copyright (c) 2009 Ilari Liusvaara
#
test_description='Test run command'
. ./test-lib.sh
cat >hello-script <<-EOF
#!$SHELL_PATH
cat hello-script
EOF
>empty
test_expect_success 'start_command reports ENOENT' '
test-run-command start-command-ENOENT ./does-not-exist
'
test_expect_success 'run_command can run a command' '
cat hello-script >hello.sh &&
chmod +x hello.sh &&
test-run-command run-command ./hello.sh >actual 2>err &&
test_cmp hello-script actual &&
test_cmp empty err
'
test_expect_success POSIXPERM 'run_command reports EACCES' '
cat hello-script >hello.sh &&
chmod -x hello.sh &&
test_must_fail test-run-command run-command ./hello.sh 2>err &&
grep "fatal: cannot exec.*hello.sh" err
'
test_expect_success POSIXPERM,SANITY 'unreadable directory in PATH' '
mkdir local-command &&
test_when_finished "chmod u+rwx local-command && rm -fr local-command" &&
git config alias.nitfol "!echo frotz" &&
chmod a-rx local-command &&
(
PATH=./local-command:$PATH &&
git nitfol >actual
) &&
echo frotz >expect &&
test_cmp expect actual
'
cat >expect <<-EOF
preloaded output of a child
Hello
World
preloaded output of a child
Hello
World
preloaded output of a child
Hello
World
preloaded output of a child
Hello
World
EOF
test_expect_success 'run_command runs in parallel with more jobs available than tasks' '
test-run-command run-command-parallel 5 sh -c "printf \"%s\n%s\n\" Hello World" 2>actual &&
test_cmp expect actual
'
test_expect_success 'run_command runs in parallel with as many jobs as tasks' '
test-run-command run-command-parallel 4 sh -c "printf \"%s\n%s\n\" Hello World" 2>actual &&
test_cmp expect actual
'
test_expect_success 'run_command runs in parallel with more tasks than jobs available' '
test-run-command run-command-parallel 3 sh -c "printf \"%s\n%s\n\" Hello World" 2>actual &&
test_cmp expect actual
'
cat >expect <<-EOF
preloaded output of a child
asking for a quick stop
preloaded output of a child
asking for a quick stop
preloaded output of a child
asking for a quick stop
EOF
test_expect_success 'run_command is asked to abort gracefully' '
test-run-command run-command-abort 3 false 2>actual &&
test_cmp expect actual
'
cat >expect <<-EOF
no further jobs available
EOF
test_expect_success 'run_command outputs ' '
test-run-command run-command-no-jobs 3 sh -c "printf \"%s\n%s\n\" Hello World" 2>actual &&
test_cmp expect actual
'
test_done