[lit] Fix shtest-format test on Windows

[modocache]

Summary

I'm not sure why, but when external_shell tests run 'cat' on a
non-existent file on my Windows environment, it *passes*. That
causes the shtest-format test to fail.

Running 'tail' on a non-existent file falis on Windows as it should, so
use that instead.

[• ddunbar]

Why don't we make this use an script we define to test the failure mode, rather than depend on an external tool, to make the test more reliable.

[delcypher]

In D27893#625978, @ddunbar wrote:

Why don't we make this use an script we define to test the failure mode, rather than depend on an external tool, to make the test more reliable.

+1

Given that Python is already a dependency we can write the script in that.

[bd1976llvm]

@ddunbar - do you mean something like:

# RUN: python -c "import os, sys; sys.exit (not os.path.isfile(sys.argv[1]))" does-not-exist

[rnk]

Personally, I think if this works, we should do it. Adding more python scripts to do simple things should be more of a last resort.

[modocache]

Sorry for the noise bringing up this change, I don't think I'll be pursuing this after all.

utils/lit/tests/Inputs/shtest-format/external_shell/fail.txt

 # Run a command that fails with error on stdout.
 #
 # RUN: echo "line 1: failed test output on stdout"
 # RUN: echo "line 2: failed test output on stdout"
-# RUN: cat "does-not-exist"
+# RUN: tail "does-not-exist"

utils/lit/tests/shtest-format.py

 # CHECK: FAIL: shtest-format :: external_shell/fail.txt
 # CHECK-NEXT: *** TEST 'shtest-format :: external_shell/fail.txt' FAILED ***
 # CHECK: Command Output (stdout):
 # CHECK-NEXT: --
 # CHECK-NEXT: line 1: failed test output on stdout
 # CHECK-NEXT: line 2: failed test output on stdout
 # CHECK: Command Output (stderr):
 # CHECK-NEXT: --
-# CHECK-NEXT: cat: does-not-exist: No such file or directory
+# CHECK-NEXT: tail: cannot open 'does-not-exist' for reading: No such file or directory
 # CHECK: --
 
 # CHECK: FAIL: shtest-format :: external_shell/fail_with_bad_encoding.txt
 # CHECK-NEXT: *** TEST 'shtest-format :: external_shell/fail_with_bad_encoding.txt' FAILED ***
 # CHECK: Command Output (stdout):
 # CHECK-NEXT: --
 # CHECK-NEXT: a line with bad encoding:
 # CHECK: --