[lit] Make internal diff work in pipelines

When using lit's internal shell, RUN lines like the following
accidentally execute an external `diff` instead of lit's internal
`diff`:

```
 # RUN: program | diff file -
 # RUN: not diff file1 file2 | FileCheck %s
```

Such cases exist now, in `clang/test/Analysis` for example.  We are
preparing patches to ensure lit's internal `diff` is called in such
cases, which will then fail because lit's internal `diff` cannot
currently be used in pipelines and doesn't recognize `-` as a
command-line option.

To enable pipelines, this patch moves lit's `diff` implementation into
an out-of-process script, similar to lit's `cat` implementation.  A
follow-up patch will implement `-` to mean stdin.

Also, when lit's `diff` prints differences to stdout in Windows, this
patch ensures it always terminate lines with `\n` not `\r\n`.  That
way, strict FileCheck directives checking the `diff` output succeed in
both Linux and Windows.  This wasn't an issue when `diff` was internal
to lit because `diff` didn't then write to the true stdout, which is
where the `\n` -> `\r\n` conversion happened in Python.

Reviewed By: probinson, stella.stamenova

Differential Revision: https://reviews.llvm.org/D66574

7 files changed, 65 insertions, 43 deletions

diff --git a/llvm/utils/lit/tests/Inputs/shtest-shell/diff-error-0.txt b/llvm/utils/lit/tests/Inputs/shtest-shell/diff-error-0.txt
deleted file mode 100644
index 81888cf8197..00000000000
--- a/llvm/utils/lit/tests/Inputs/shtest-shell/diff-error-0.txt
+++ /dev/null
@@ -1,3 +0,0 @@
-# Check error on a unsupported diff (cannot be part of a pipeline).
-#
-# RUN: diff diff-error-0.txt diff-error-0.txt | echo Output
diff --git a/llvm/utils/lit/tests/Inputs/shtest-shell/diff-pipes.txt b/llvm/utils/lit/tests/Inputs/shtest-shell/diff-pipes.txt
new file mode 100644
index 00000000000..ce0abca1661
--- /dev/null
+++ b/llvm/utils/lit/tests/Inputs/shtest-shell/diff-pipes.txt
@@ -0,0 +1,15 @@
+# RUN: echo foo > %t.foo
+# RUN: echo bar > %t.bar
+
+# Check output pipe.
+# RUN: diff %t.foo %t.foo | FileCheck -allow-empty -check-prefix=EMPTY %s
+# RUN: diff -u %t.foo %t.bar | FileCheck %s && false || true
+
+# Fail so lit will print output.
+# RUN: false
+
+# CHECK:      @@
+# CHECK-NEXT: -foo
+# CHECK-NEXT: +bar
+
+# EMPTY-NOT: {{.}}
diff --git a/llvm/utils/lit/tests/Inputs/shtest-shell/diff-unified-error-0.txt b/llvm/utils/lit/tests/Inputs/shtest-shell/diff-unified-error-0.txt
deleted file mode 100644
index ee3858a02ba..00000000000
--- a/llvm/utils/lit/tests/Inputs/shtest-shell/diff-unified-error-0.txt
+++ /dev/null
@@ -1,3 +0,0 @@
-# Check bad -U argument.
-# RUN: echo foo > %t
-# RUN: diff -U 30.1 %t %t
diff --git a/llvm/utils/lit/tests/Inputs/shtest-shell/diff-unified-error-1.txt b/llvm/utils/lit/tests/Inputs/shtest-shell/diff-unified-error-1.txt
deleted file mode 100644
index e46e8b5783b..00000000000
--- a/llvm/utils/lit/tests/Inputs/shtest-shell/diff-unified-error-1.txt
+++ /dev/null
@@ -1,3 +0,0 @@
-# Check bad -U argument.
-# RUN: echo foo > %t
-# RUN: diff -U-1 %t %t
diff --git a/llvm/utils/lit/tests/Inputs/shtest-shell/diff-unified.txt b/llvm/utils/lit/tests/Inputs/shtest-shell/diff-unified.txt
index a6a7eef3f9b..e977c193247 100644
--- a/llvm/utils/lit/tests/Inputs/shtest-shell/diff-unified.txt
+++ b/llvm/utils/lit/tests/Inputs/shtest-shell/diff-unified.txt
@@ -30,5 +30,9 @@
 # RUN: diff -U4 %t.foo %t.bar && false || true
 # RUN: diff -U0 %t.foo %t.bar && false || true
 
+# Check bad -U argument.
+# RUN: diff -U 30.1 %t.foo %t.foo && false || true
+# RUN: diff -U-1 %t.foo %t.foo && false || true
+
 # Fail so lit will print output.
 # RUN: false
diff --git a/llvm/utils/lit/tests/max-failures.py b/llvm/utils/lit/tests/max-failures.py
index f661980ac2b..3b85ae18611 100644
--- a/llvm/utils/lit/tests/max-failures.py
+++ b/llvm/utils/lit/tests/max-failures.py
@@ -8,7 +8,7 @@
 #
 # END.
 
-# CHECK: Failing Tests (33)
+# CHECK: Failing Tests (31)
 # CHECK: Failing Tests (1)
 # CHECK: Failing Tests (2)
 # CHECK: error: argument --max-failures: requires positive integer, but found '0'
diff --git a/llvm/utils/lit/tests/shtest-shell.py b/llvm/utils/lit/tests/shtest-shell.py
index cef3728dd00..150c2fe362c 100644
--- a/llvm/utils/lit/tests/shtest-shell.py
+++ b/llvm/utils/lit/tests/shtest-shell.py
@@ -86,28 +86,20 @@
 # CHECK: ***
 
 
-# CHECK: FAIL: shtest-shell :: diff-error-0.txt
-# CHECK: *** TEST 'shtest-shell :: diff-error-0.txt' FAILED ***
-# CHECK: $ "diff" "diff-error-0.txt" "diff-error-0.txt"
-# CHECK: # command stderr:
-# CHECK: Unsupported: 'diff' cannot be part of a pipeline
-# CHECK: error: command failed with exit status: 127
-# CHECK: ***
-
 # CHECK: FAIL: shtest-shell :: diff-error-1.txt
 # CHECK: *** TEST 'shtest-shell :: diff-error-1.txt' FAILED ***
 # CHECK: $ "diff" "-B" "temp1.txt" "temp2.txt"
 # CHECK: # command stderr:
 # CHECK: Unsupported: 'diff': option -B not recognized
-# CHECK: error: command failed with exit status: 127
+# CHECK: error: command failed with exit status: 1
 # CHECK: ***
 
 # CHECK: FAIL: shtest-shell :: diff-error-2.txt
 # CHECK: *** TEST 'shtest-shell :: diff-error-2.txt' FAILED ***
 # CHECK: $ "diff" "temp.txt"
 # CHECK: # command stderr:
-# CHECK: Error:  missing or extra operand
-# CHECK: error: command failed with exit status: 127
+# CHECK: Error: missing or extra operand
+# CHECK: error: command failed with exit status: 1
 # CHECK: ***
 
 # CHECK: FAIL: shtest-shell :: diff-error-3.txt
@@ -134,18 +126,43 @@
 # CHECK: *** TEST 'shtest-shell :: diff-error-5.txt' FAILED ***
 # CHECK: $ "diff"
 # CHECK: # command stderr:
-# CHECK: Error:  missing or extra operand
-# CHECK: error: command failed with exit status: 127
+# CHECK: Error: missing or extra operand
+# CHECK: error: command failed with exit status: 1
 # CHECK: ***
 
 # CHECK: FAIL: shtest-shell :: diff-error-6.txt
 # CHECK: *** TEST 'shtest-shell :: diff-error-6.txt' FAILED ***
 # CHECK: $ "diff"
 # CHECK: # command stderr:
-# CHECK: Error:  missing or extra operand
-# CHECK: error: command failed with exit status: 127
+# CHECK: Error: missing or extra operand
+# CHECK: error: command failed with exit status: 1
+# CHECK: ***
+
+
+# CHECK: FAIL: shtest-shell :: diff-pipes.txt
+
+# CHECK: *** TEST 'shtest-shell :: diff-pipes.txt' FAILED ***
+
+# CHECK: $ "diff" "{{[^"]*}}.foo" "{{[^"]*}}.foo"
+# CHECK-NOT: note
+# CHECK-NOT: error
+# CHECK: $ "FileCheck"
+# CHECK-NOT: note
+# CHECK-NOT: error
+
+# CHECK: $ "diff" "-u" "{{[^"]*}}.foo" "{{[^"]*}}.bar"
+# CHECK: note: command had no output on stdout or stderr
+# CHECK: error: command failed with exit status: 1
+# CHECK: $ "FileCheck"
+# CHECK-NOT: note
+# CHECK-NOT: error
+# CHECK: $ "true"
+
+# CHECK: $ "false"
+
 # CHECK: ***
 
+
 # CHECK: FAIL: shtest-shell :: diff-r-error-0.txt
 # CHECK: *** TEST 'shtest-shell :: diff-r-error-0.txt' FAILED ***
 # CHECK: $ "diff" "-r" 
@@ -255,23 +272,6 @@
 # CHECK: ***
 
 
-# CHECK: FAIL: shtest-shell :: diff-unified-error-0.txt
-# CHECK: *** TEST 'shtest-shell :: diff-unified-error-0.txt' FAILED ***
-# CHECK: $ "diff" "-U" "30.1" "{{[^"]*}}" "{{[^"]*}}"
-# CHECK: # command stderr:
-# CHECK: Error: invalid '-U' argument: 30.1
-# CHECK: error: command failed with exit status: 127
-# CHECK: ***
-
-# CHECK: FAIL: shtest-shell :: diff-unified-error-1.txt
-# CHECK: *** TEST 'shtest-shell :: diff-unified-error-1.txt' FAILED ***
-# CHECK: $ "diff" "-U-1" "{{[^"]*}}" "{{[^"]*}}"
-# CHECK: # command stderr:
-# CHECK: Error: invalid '-U' argument: -1
-# CHECK: error: command failed with exit status: 127
-# CHECK: ***
-
-
 # CHECK: FAIL: shtest-shell :: diff-unified.txt
 
 # CHECK: *** TEST 'shtest-shell :: diff-unified.txt' FAILED ***
@@ -330,6 +330,18 @@
 # CHECK-NEXT: error: command failed with exit status: 1
 # CHECK-NEXT: $ "true"
 
+# CHECK: $ "diff" "-U" "30.1" "{{[^"]*}}" "{{[^"]*}}"
+# CHECK: # command stderr:
+# CHECK: Error: invalid '-U' argument: 30.1
+# CHECK: error: command failed with exit status: 1
+# CHECK: $ "true"
+
+# CHECK: $ "diff" "-U-1" "{{[^"]*}}" "{{[^"]*}}"
+# CHECK: # command stderr:
+# CHECK: Error: invalid '-U' argument: -1
+# CHECK: error: command failed with exit status: 1
+# CHECK: $ "true"
+
 # CHECK: $ "false"
 
 # CHECK: ***
@@ -426,4 +438,4 @@
 # CHECK: ***
 
 # CHECK: PASS: shtest-shell :: valid-shell.txt
-# CHECK: Failing Tests (33)
+# CHECK: Failing Tests (31)