From a4b18382ad60383663ac9ec12194a9cfd7a6b1f1 Mon Sep 17 00:00:00 2001
From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Tue, 14 Jan 2020 23:33:01 +0100
Subject: [PATCH] mingw: do not treat `COM0` as a reserved file name

In 4dc42c6c186 (mingw: refuse paths containing reserved names,
2019-12-21), we started disallowing file names that are reserved, e.g.
`NUL`, `CONOUT$`, etc.

This included `COM<n>` where `<n>` is a digit. Unfortunately, this
includes `COM0` but only `COM1`, ..., `COM9` are reserved, according to
the official documentation, `COM0` is mentioned in the "NT Namespaces"
section but it is explicitly _omitted_ from the list of reserved names:
https://docs.microsoft.com/en-us/windows/win32/fileio/naming-a-file#naming-conventions

Tests corroborate this: it is totally possible to write a file called
`com0.c` on Windows 10, but not `com1.c`.

So let's tighten the code to disallow only the reserved `COM<n>` file
names, but to allow `COM0` again.

This fixes https://github.com/git-for-windows/git/issues/2470.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 compat/mingw.c        | 8 +++++---
 t/t0060-path-utils.sh | 2 ++
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/compat/mingw.c b/compat/mingw.c
index ac02ec1312b15d..f5de482c54589b 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -3231,12 +3231,14 @@ int is_valid_win32_path(const char *path, int allow_literal_nul)
 					continue;
 				}
 				break;
-			case 'c': case 'C': /* COM<N>, CON, CONIN$, CONOUT$ */
+			case 'c': case 'C':
+				/* COM1 ... COM9, CON, CONIN$, CONOUT$ */
 				if ((c = path[++i]) != 'o' && c != 'O')
 					goto not_a_reserved_name;
 				c = path[++i];
-				if (c == 'm' || c == 'M') { /* COM<N> */
-					if (!isdigit(path[++i]))
+				if (c == 'm' || c == 'M') { /* COM1 ... COM9 */
+					c = path[++i];
+					if (c < '1' || c > '9')
 						goto not_a_reserved_name;
 				} else if (c == 'n' || c == 'N') { /* CON */
 					c = path[i + 1];
diff --git a/t/t0060-path-utils.sh b/t/t0060-path-utils.sh
index 28dc044ad09c5c..7aec9b74640141 100755
--- a/t/t0060-path-utils.sh
+++ b/t/t0060-path-utils.sh
@@ -476,6 +476,7 @@ test_expect_success MINGW 'is_valid_path() on Windows' '
 		C:\\git \
 		comm \
 		conout.c \
+		com0.c \
 		lptN \
 		\
 		--not \
@@ -488,6 +489,7 @@ test_expect_success MINGW 'is_valid_path() on Windows' '
 		"AUX.c" \
 		"abc/conOut\$  .xyz/test" \
 		lpt8 \
+		com9.c \
 		"lpt*" \
 		Nul \
 		"PRN./abc"