git p4: work around p4 bug that causes empty symlinks

Damien Gérard highlights an interesting problem.  Some p4
repositories end up with symlinks that have an empty target.  It
is not possible to create this with current p4, but they do
indeed exist.

The effect in git p4 is that "p4 print" on the symlink returns an
empty string, confusing the curret symlink-handling code.

Such broken repositories cause problems in p4 as well, even with
no git involved.  In p4, syncing to a change that includes a
bogus symlink causes errors:

    //depot/empty-symlink - updating /home/me/p4/empty-symlink
    rename: /home/me/p4/empty-symlink: No such file or directory

and leaves no symlink.

In git, replicate the p4 behavior by ignoring these bad symlinks.
If, in a later p4 revision, the symlink happens to point to
something non-null, the symlink will be replaced properly.

Add a big test for all this too.

This happens to be a regression introduced by 1292df1 (git-p4:
Fix occasional truncation of symlink contents., 2013-08-08) and
appeared first in 1.8.5.  But it shows up only in p4 repositories
of dubious character, so can wait for a proper release.

Tested-by: Damien Gérard <damien@iwi.me>
Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -2075,7 +2075,14 @@ def streamOneP4File(self, file, contents):
             # p4 print on a symlink sometimes contains "target\n";
             # if it does, remove the newline
             data = ''.join(contents)
-            if data[-1] == '\n':
+            if not data:
+                # Some version of p4 allowed creating a symlink that pointed
+                # to nothing.  This causes p4 errors when checking out such
+                # a change, and errors here too.  Work around it by ignoring
+                # the bad symlink; hopefully a future change fixes it.
+                print "\nIgnoring empty symlink in %s" % file['depotFile']
+                return
+            elif data[-1] == '\n':
                 contents = [data[:-1]]
             else:
                 contents = [data]

t/t9802-git-p4-filetype.sh

@@ -267,6 +267,72 @@ test_expect_success SYMLINKS 'ensure p4 symlink parsed correctly' '
 	)
 '
 
+test_expect_success SYMLINKS 'empty symlink target' '
+	(
+		# first create the file as a file
+		cd "$cli" &&
+		>empty-symlink &&
+		p4 add empty-symlink &&
+		p4 submit -d "add empty-symlink as a file"
+	) &&
+	(
+		# now change it to be a symlink to "target1"
+		cd "$cli" &&
+		p4 edit empty-symlink &&
+		p4 reopen -t symlink empty-symlink &&
+		rm empty-symlink &&
+		ln -s target1 empty-symlink &&
+		p4 add empty-symlink &&
+		p4 submit -d "make empty-symlink point to target1"
+	) &&
+	(
+		# Hack the p4 depot to make the symlink point to nothing;
+		# this should not happen in reality, but shows up
+		# in p4 repos in the wild.
+		#
+		# The sed expression changes this:
+		#     @@
+		#     text
+		#     @target1
+		#     @
+		# to this:
+		#     @@
+		#     text
+		#     @@
+		#
+		cd "$db/depot" &&
+		sed "/@target1/{; s/target1/@/; n; d; }" \
+		    empty-symlink,v >empty-symlink,v.tmp &&
+		mv empty-symlink,v.tmp empty-symlink,v
+	) &&
+	(
+		# Make sure symlink really is empty.  Asking
+		# p4 to sync here will make it generate errors.
+		cd "$cli" &&
+		p4 print -q //depot/empty-symlink#2 >out &&
+		test ! -s out
+	) &&
+	test_when_finished cleanup_git &&
+
+	# make sure git p4 handles it without error
+	git p4 clone --dest="$git" //depot@all &&
+
+	# fix the symlink, make it point to "target2"
+	(
+		cd "$cli" &&
+		p4 open empty-symlink &&
+		rm empty-symlink &&
+		ln -s target2 empty-symlink &&
+		p4 submit -d "make empty-symlink point to target2"
+	) &&
+	cleanup_git &&
+	git p4 clone --dest="$git" //depot@all &&
+	(
+		cd "$git" &&
+		test $(readlink empty-symlink) = target2
+	)
+'
+
 test_expect_success 'kill p4d' '
 	kill_p4d
 '