[PATCH] mingw: do not treat `COM0` as a reserved file name

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

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>
---
    mingw: handle COM0 correctly
    
    The code to prevent the reserved COM<n> file names erred on the side of
    simple code, and sadly sacrificed correctness while at it. This fixes
    it.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-754%2Fdscho%2Fcom0-is-not-a-reserved-name-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-754/dscho/com0-is-not-a-reserved-name-v1
Pull-Request: https://github.com/git/git/pull/754

 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 d14065d60ec..835a340211e 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -2581,12 +2581,14 @@ 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 2ea2d00c39a..56db5c8abab 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"

base-commit: 9fadedd637b312089337d73c3ed8447e9f0aa775
-- 
gitgitgadget