run-command: be helpful with Git LFS fails on Windows 7

Git LFS is now built with Go 1.21 which no longer supports Windows 7.
However, Git for Windows still wants to support Windows 7.

Ideally, Git LFS would re-introduce Windows 7 support until Git for
Windows drops support for Windows 7, but that's not going to happen:
git-for-windows#4996 (comment)

The next best thing we can do is to let the users know what is
happening, and how to get out of their fix, at least.

This is not quite as easy as it would first seem because programs
compiled with Go 1.21 or newer will simply throw an exception and fail
with an Access Violation on Windows 7.

The only way I found to address this is to replicate the logic from Go's
very own `version` command (which can determine the Go version with
which a given executable was built) to detect the situation, and in that
case offer a helpful error message.

This addresses git-for-windows#4996.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/win32/path-utils.c

@@ -1,4 +1,7 @@
 #include "../../git-compat-util.h"
+#include "../../wrapper.h"
+#include "../../strbuf.h"
+#include "../../versioncmp.h"
 
 int win32_has_dos_drive_prefix(const char *path)
 {
@@ -50,3 +53,195 @@ int win32_offset_1st_component(const char *path)
 
 	return pos + is_dir_sep(*pos) - path;
 }
+
+static int read_at(int fd, char *buffer, size_t offset, size_t size)
+{
+	if (lseek(fd, offset, SEEK_SET) < 0) {
+		fprintf(stderr, "could not seek to 0x%x\n", (unsigned int)offset);
+		return -1;
+	}
+
+	return read_in_full(fd, buffer, size);
+}
+
+static size_t le16(const char *buffer)
+{
+	unsigned char *u = (unsigned char *)buffer;
+	return u[0] | (u[1] << 8);
+}
+
+static size_t le32(const char *buffer)
+{
+	return le16(buffer) | (le16(buffer + 2) << 16);
+}
+
+/*
+ * Determine the Go version of a given executable, if it was built with Go.
+ *
+ * This recapitulates the logic from
+ * https://github.com/golang/go/blob/master/src/cmd/go/internal/version/version.go
+ * (without requiring the user to install `go.exe` to find out).
+ */
+static ssize_t get_go_version(const char *path, char *go_version, size_t go_version_size)
+{
+	int fd = open(path, O_RDONLY);
+	char buffer[1024];
+	off_t offset;
+	size_t num_sections, opt_header_size, i;
+	char *p = NULL, *q;
+	ssize_t res = -1;
+
+	if (fd < 0)
+		return -1;
+
+	if (read_in_full(fd, buffer, 2) < 0)
+		goto fail;
+
+	/*
+	 * Parse the PE file format, for more details, see
+	 * https://en.wikipedia.org/wiki/Portable_Executable#Layout and
+	 * https://learn.microsoft.com/en-us/windows/win32/debug/pe-format
+	 */
+	if (buffer[0] != 'M' || buffer[1] != 'Z')
+		goto fail;
+
+	if (read_at(fd, buffer, 0x3c, 4) < 0)
+		goto fail;
+
+	/* Read the `PE\0\0` signature and the COFF file header */
+	offset = le32(buffer);
+	if (read_at(fd, buffer, offset, 24) < 0)
+		goto fail;
+
+	if (buffer[0] != 'P' || buffer[1] != 'E' || buffer[2] != '\0' || buffer[3] != '\0')
+		goto fail;
+
+	num_sections = le16(buffer + 6);
+	opt_header_size = le16(buffer + 20);
+	offset += 24; /* skip file header */
+
+	/*
+	 * Validate magic number 0x10b or 0x20b, for full details see
+	 * https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#optional-header-standard-fields-image-only
+	 */
+	if (read_at(fd, buffer, offset, 2) < 0 ||
+	    ((i = le16(buffer)) != 0x10b && i != 0x20b))
+		goto fail;
+
+	offset += opt_header_size;
+
+	for (i = 0; i < num_sections; i++) {
+		if (read_at(fd, buffer, offset + i * 40, 40) < 0)
+			goto fail;
+
+		/*
+		 * For full details about the section headers, see
+		 * https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#section-table-section-headers
+		 */
+		if ((le32(buffer + 36) /* characteristics */ & ~0x600000) /* IMAGE_SCN_ALIGN_32BYTES */ ==
+		    (/* IMAGE_SCN_CNT_INITIALIZED_DATA */ 0x00000040 |
+		     /* IMAGE_SCN_MEM_READ */ 0x40000000 |
+		     /* IMAGE_SCN_MEM_WRITE */ 0x80000000)) {
+			size_t size = le32(buffer + 16); /* "SizeOfRawData " */
+			size_t pointer = le32(buffer + 20); /* "PointerToRawData " */
+
+			/*
+			 * Skip the section if either size or pointer is 0, see
+			 * https://github.com/golang/go/blob/go1.21.0/src/debug/buildinfo/buildinfo.go#L333
+			 * for full details.
+			 *
+			 * Merely seeing a non-zero size will not actually do,
+			 * though: he size must be at least `buildInfoSize`,
+			 * i.e. 32, and we expect a UVarint (at least another
+			 * byte) _and_ the bytes representing the string,
+			 * which we expect to start with the letters "go" and
+			 * continue with the Go version number.
+			 */
+			if (size < 32 + 1 + 2 + 1 || !pointer)
+				continue;
+
+			p = malloc(size);
+
+			if (!p || read_at(fd, p, pointer, size) < 0)
+				goto fail;
+
+			/*
+			 * Look for the build information embedded by Go, see
+			 * https://github.com/golang/go/blob/go1.21.0/src/debug/buildinfo/buildinfo.go#L165-L175
+			 * for full details.
+			 *
+			 * Note: Go contains code to enforce alignment along a
+			 * 16-byte boundary. In practice, no `.exe` has been
+			 * observed that required any adjustment, therefore
+			 * this here code skips that logic for simplicity.
+			 */
+			q = memmem(p, size - 18, "\xff Go buildinf:", 14);
+			if (!q)
+				goto fail;
+			/*
+			 * Decode the build blob. For full details, see
+			 * https://github.com/golang/go/blob/go1.21.0/src/debug/buildinfo/buildinfo.go#L177-L191
+			 *
+			 * Note: The `endianness` values observed in practice
+			 * were always 2, therefore the complex logic to handle
+			 * any other value is skipped for simplicty.
+			 */
+			if ((q[14] == 8 || q[14] == 4) && q[15] == 2) {
+				/*
+				 * Only handle a Go version string with fewer
+				 * than 128 characters, so the Go UVarint at
+				 * q[32] that indicates the string's length must
+				 * be only one byte (without the high bit set).
+				 */
+				if ((q[32] & 0x80) ||
+				    !q[32] ||
+				    (q + 33 + q[32] - p) > size ||
+				    q[32] + 1 > go_version_size)
+					goto fail;
+				res = q[32];
+				memcpy(go_version, q + 33, res);
+				go_version[res] = '\0';
+				break;
+			}
+		}
+	}
+
+fail:
+	free(p);
+	close(fd);
+	return res;
+}
+
+void win32_warn_about_git_lfs_on_windows7(int exit_code, const char *argv0)
+{
+	char buffer[128], *git_lfs = NULL;
+	const char *p;
+
+	/*
+	 * Git LFS v3.5.1 fails with an Access Violation on Windows 7; That
+	 * would usually show up as an exit code 0xc0000005. For some reason
+	 * (probably because at this point, we no longer have the _original_
+	 * HANDLE that was returned by `CreateProcess()`) we get 0xb00 instead.
+	 */
+	if (exit_code != 0x0b00)
+		return;
+	if (GetVersion() >> 16 > 7601)
+		return; /* Warn only on Windows 7 or older */
+	if (!starts_with(argv0, "git-lfs ") ||
+	    !(git_lfs = locate_in_PATH("git-lfs")))
+		return;
+	if (get_go_version(git_lfs, buffer, sizeof(buffer)) > 0 &&
+	    skip_prefix(buffer, "go", &p) &&
+	    versioncmp("1.21.0", p) <= 0)
+		warning("This program was built with Go v%s\n"
+			"i.e. without support for this Windows version:\n"
+			"\n\t%s\n"
+			"\n"
+			"To work around this, you can download and install a "
+			"working version from\n"
+			"\n"
+			"\thttps://github.com/git-lfs/git-lfs/releases/tag/"
+			"v3.4.1\n",
+			p, git_lfs);
+	free(git_lfs);
+}

compat/win32/path-utils.h

@@ -30,4 +30,7 @@ static inline int win32_has_dir_sep(const char *path)
 int win32_offset_1st_component(const char *path);
 #define offset_1st_component win32_offset_1st_component
 
+void win32_warn_about_git_lfs_on_windows7(int exit_code, const char *argv0);
+#define warn_about_git_lfs_on_windows7 win32_warn_about_git_lfs_on_windows7
+
 #endif

git-compat-util.h

@@ -520,6 +520,12 @@ static inline int git_offset_1st_component(const char *path)
 #define offset_1st_component git_offset_1st_component
 #endif
 
+#ifndef warn_about_git_lfs_on_windows7
+static inline void warn_about_git_lfs_on_windows7(int exit_code, const char *argv0)
+{
+}
+#endif
+
 #ifndef is_valid_path
 #define is_valid_path(path) 1
 #endif

run-command.c

@@ -566,6 +566,7 @@ static int wait_or_whine(pid_t pid, const char *argv0, int in_signal)
 		 */
 		code += 128;
 	} else if (WIFEXITED(status)) {
+		warn_about_git_lfs_on_windows7(status, argv0);
 		code = WEXITSTATUS(status);
 	} else {
 		if (!in_signal)