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

From: Johannes Schindelin <[email protected]>

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.

---
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(-)

index d14065d60ec..835a340211e 100644
@@ -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];

index 2ea2d00c39a..56db5c8abab 100755
@@ -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

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