tests: A SANITY test prereq for testing if we're root

Some tests depend on not being able to write to files after chmod
-w. This doesn't work when running the tests as root.

Change test-lib.sh to test if this works, and if so it sets a new
SANITY test prerequisite. The tests that use this previously failed
when run under root.

There was already a test for this in t3600-rm.sh, added by Junio C
Hamano in 2283645 in 2006. That check now uses the new SANITY
prerequisite.

Some of this was resurrected from the "Tests in Cygwin" thread in May
2009:

    http://thread.gmane.org/gmane.comp.version-control.git/116729/focus=118385

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/README

@@ -523,6 +523,11 @@ use these, and "test_set_prereq" for how to define your own.
    The filesystem we're on supports symbolic links. E.g. a FAT
    filesystem doesn't support these. See 704a3143 for details.
 
+ - SANITY
+
+   Test is not run by root user, and an attempt to write to an
+   unwritable file is expected to fail correctly.
+
 Tips for Writing Tests
 ----------------------
 

t/t0001-init.sh

@@ -301,7 +301,7 @@ test_expect_success 'init notices EEXIST (2)' '
 	)
 '
 
-test_expect_success POSIXPERM 'init notices EPERM' '
+test_expect_success POSIXPERM,SANITY 'init notices EPERM' '
 	rm -fr newdir &&
 	(
 		mkdir newdir &&

t/t0004-unwritable.sh

@@ -15,7 +15,7 @@ test_expect_success setup '
 
 '
 
-test_expect_success POSIXPERM 'write-tree should notice unwritable repository' '
+test_expect_success POSIXPERM,SANITY 'write-tree should notice unwritable repository' '
 
 	(
 		chmod a-w .git/objects .git/objects/?? &&
@@ -27,7 +27,7 @@ test_expect_success POSIXPERM 'write-tree should notice unwritable repository' '
 
 '
 
-test_expect_success POSIXPERM 'commit should notice unwritable repository' '
+test_expect_success POSIXPERM,SANITY 'commit should notice unwritable repository' '
 
 	(
 		chmod a-w .git/objects .git/objects/?? &&
@@ -39,7 +39,7 @@ test_expect_success POSIXPERM 'commit should notice unwritable repository' '
 
 '
 
-test_expect_success POSIXPERM 'update-index should notice unwritable repository' '
+test_expect_success POSIXPERM,SANITY 'update-index should notice unwritable repository' '
 
 	(
 		echo 6O >file &&
@@ -52,7 +52,7 @@ test_expect_success POSIXPERM 'update-index should notice unwritable repository'
 
 '
 
-test_expect_success POSIXPERM 'add should notice unwritable repository' '
+test_expect_success POSIXPERM,SANITY 'add should notice unwritable repository' '
 
 	(
 		echo b >file &&

t/t1004-read-tree-m-u-wf.sh

@@ -177,7 +177,7 @@ test_expect_success SYMLINKS 'funny symlink in work tree' '
 
 '
 
-test_expect_success SYMLINKS 'funny symlink in work tree, un-unlink-able' '
+test_expect_success SYMLINKS,SANITY 'funny symlink in work tree, un-unlink-able' '
 
 	rm -fr a b &&
 	git reset --hard &&

t/t3600-rm.sh

@@ -28,22 +28,6 @@ embedded' &&
      git commit -m 'add files with tabs and newlines'
 "
 
-# Determine rm behavior
-# Later we will try removing an unremovable path to make sure
-# git rm barfs, but if the test is run as root that cannot be
-# arranged.
-: >test-file
-chmod a-w .
-rm -f test-file 2>/dev/null
-if test -f test-file
-then
-	test_set_prereq RO_DIR
-else
-	skip_all='skipping removal failure test (perhaps running as root?)'
-fi
-chmod 775 .
-rm -f test-file
-
 test_expect_success \
     'Pre-check that foo exists and is in index before git rm foo' \
     '[ -f foo ] && git ls-files --error-unmatch foo'

t/t3700-add.sh

@@ -179,7 +179,7 @@ test_expect_success 'git add --refresh' '
 	test -z "`git diff-index HEAD -- foo`"
 '
 
-test_expect_success POSIXPERM 'git add should fail atomically upon an unreadable file' '
+test_expect_success POSIXPERM,SANITY 'git add should fail atomically upon an unreadable file' '
 	git reset --hard &&
 	date >foo1 &&
 	date >foo2 &&
@@ -190,7 +190,7 @@ test_expect_success POSIXPERM 'git add should fail atomically upon an unreadable
 
 rm -f foo2
 
-test_expect_success POSIXPERM 'git add --ignore-errors' '
+test_expect_success POSIXPERM,SANITY 'git add --ignore-errors' '
 	git reset --hard &&
 	date >foo1 &&
 	date >foo2 &&
@@ -201,7 +201,7 @@ test_expect_success POSIXPERM 'git add --ignore-errors' '
 
 rm -f foo2
 
-test_expect_success POSIXPERM 'git add (add.ignore-errors)' '
+test_expect_success POSIXPERM,SANITY 'git add (add.ignore-errors)' '
 	git config add.ignore-errors 1 &&
 	git reset --hard &&
 	date >foo1 &&
@@ -212,7 +212,7 @@ test_expect_success POSIXPERM 'git add (add.ignore-errors)' '
 '
 rm -f foo2
 
-test_expect_success POSIXPERM 'git add (add.ignore-errors = false)' '
+test_expect_success POSIXPERM,SANITY 'git add (add.ignore-errors = false)' '
 	git config add.ignore-errors 0 &&
 	git reset --hard &&
 	date >foo1 &&
@@ -223,7 +223,7 @@ test_expect_success POSIXPERM 'git add (add.ignore-errors = false)' '
 '
 rm -f foo2
 
-test_expect_success POSIXPERM '--no-ignore-errors overrides config' '
+test_expect_success POSIXPERM,SANITY '--no-ignore-errors overrides config' '
        git config add.ignore-errors 1 &&
        git reset --hard &&
        date >foo1 &&

t/t7300-clean.sh

@@ -388,16 +388,15 @@ test_expect_success 'core.excludesfile' '
 
 '
 
-test_expect_success 'removal failure' '
+test_expect_success SANITY 'removal failure' '
 
 	mkdir foo &&
 	touch foo/bar &&
 	(exec <foo/bar &&
 	 chmod 0 foo &&
-	 test_must_fail git clean -f -d)
-
+	 test_must_fail git clean -f -d &&
+	 chmod 755 foo)
 '
-chmod 755 foo
 
 test_expect_success 'nested git work tree' '
 	rm -fr foo bar &&

t/t7508-status.sh

@@ -793,7 +793,7 @@ test_expect_success 'commit --dry-run submodule summary (--amend)' '
 	test_cmp expect output
 '
 
-test_expect_success POSIXPERM 'status succeeds in a read-only repository' '
+test_expect_success POSIXPERM,SANITY 'status succeeds in a read-only repository' '
 	(
 		chmod a-w .git &&
 		# make dir1/tracked stat-dirty

t/test-lib.sh

@@ -897,3 +897,7 @@ test -z "$NO_PYTHON" && test_set_prereq PYTHON
 # test whether the filesystem supports symbolic links
 ln -s x y 2>/dev/null && test -h y 2>/dev/null && test_set_prereq SYMLINKS
 rm -f y
+
+# When the tests are run as root, permission tests will report that
+# things are writable when they shouldn't be.
+test -w / || test_set_prereq SANITY