git-p4: preserve utf8 BOM when importing from p4 to git

Perforce has a file type "utf8" which represents a text file with
explicit BOM. utf8-encoded files *without* BOM are stored as
regular file type "text". The "utf8" file type behaves like text
in all but one important way: it is stored, internally, without
the leading 3 BOM bytes.

git-p4 has historically imported utf8-with-BOM files (files stored,
in Perforce, as type "utf8") the same way as regular text files -
losing the BOM in the process.

Under most circumstances this issue has little functional impact,
as most systems consider the BOM to be optional and redundant, but
this *is* a correctness failure, and can have lead to practical
issues for example when BOMs are explicitly included in test files,
for example in a file encoding test suite.

Fix the handling of utf8-with-BOM files when importing changes from
p4 to git, and introduce a test that checks it is working correctly.

Signed-off-by: Tao Klerks <tao@klerks.biz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -2885,6 +2885,16 @@ class P4Sync(Command, P4UserMap):
             print("\nIgnoring apple filetype file %s" % file['depotFile'])
             return
 
+        if type_base == "utf8":
+            # The type utf8 explicitly means utf8 *with BOM*. These are
+            # streamed just like regular text files, however, without
+            # the BOM in the stream.
+            # Therefore, to accurately import these files into git, we
+            # need to explicitly re-add the BOM before writing.
+            # 'contents' is a set of bytes in this case, so create the
+            # BOM prefix as a b'' literal.
+            contents = [b'\xef\xbb\xbf' + contents[0]] + contents[1:]
+
         # Note that we do not try to de-mangle keywords on utf16 files,
         # even though in theory somebody may want that.
         regexp = p4_keywords_regexp_for_type(type_base, type_mods)

t/t9802-git-p4-filetype.sh

@@ -333,4 +333,38 @@ test_expect_success SYMLINKS 'empty symlink target' '
 	)
 '
 
+test_expect_success SYMLINKS 'utf-8 with and without BOM in text file' '
+	(
+		cd "$cli" &&
+
+		# some utf8 content
+		echo some tǣxt >utf8-nobom-test &&
+
+		# same utf8 content as before but with bom
+		echo some tǣxt | sed '\''s/^/\xef\xbb\xbf/'\'' >utf8-bom-test &&
+
+		# bom only
+		dd bs=1 count=3 if=utf8-bom-test of=utf8-bom-empty-test &&
+
+		p4 add utf8-nobom-test utf8-bom-test utf8-bom-empty-test &&
+		p4 submit -d "add utf8 test files"
+	) &&
+	test_when_finished cleanup_git &&
+
+	git p4 clone --dest="$git" //depot@all &&
+	(
+		cd "$git" &&
+		git checkout refs/remotes/p4/master &&
+
+		echo some tǣxt >utf8-nobom-check &&
+		test_cmp utf8-nobom-check utf8-nobom-test &&
+
+		echo some tǣxt | sed '\''s/^/\xef\xbb\xbf/'\'' >utf8-bom-check &&
+		test_cmp utf8-bom-check utf8-bom-test &&
+
+		dd bs=1 count=3 if=utf8-bom-check of=utf8-bom-empty-check &&
+		test_cmp utf8-bom-empty-check utf8-bom-empty-test
+	)
+'
+
 test_done