build: generate a version number, show in log on startup

src/build/Version.zig

@@ -0,0 +1,53 @@
+const Version = @This();
+
+const std = @import("std");
+
+/// The short hash (7 characters) of the latest commit.
+short_hash: []const u8,
+
+/// True if there was a diff at build time.
+changes: bool,
+
+/// The tag -- if any -- that this commit is a part of.
+tag: ?[]const u8,
+
+/// The branch that was checked out at the time of the build.
+branch: []const u8,
+
+/// Initialize the version and detect it from the Git environment. This
+/// allocates using the build allocator and doesn't free.
+pub fn detect(b: *std.Build) !Version {
+    // Execute a bunch of git commands to determine the automatic version.
+    var code: u8 = 0;
+    const branch = try b.execAllowFail(&[_][]const u8{ "git", "-C", b.build_root.path orelse ".", "rev-parse", "--abbrev-ref", "HEAD" }, &code, .Ignore);
+
+    const short_hash = short_hash: {
+        const output = try b.execAllowFail(&[_][]const u8{ "git", "-C", b.build_root.path orelse ".", "log", "--pretty=format:%h", "-n", "1" }, &code, .Ignore);
+        break :short_hash std.mem.trimRight(u8, output, "\r\n ");
+    };
+
+    const tag = b.execAllowFail(&[_][]const u8{ "git", "-C", b.build_root.path orelse ".", "describe", "--exact-match", "--tags" }, &code, .Ignore) catch |err| switch (err) {
+        error.ExitCodeFailure => "", // expected
+        else => return err,
+    };
+
+    _ = b.execAllowFail(&[_][]const u8{
+        "git",
+        "-C",
+        b.build_root.path orelse ".",
+        "diff",
+        "--quiet",
+        "--exit-code",
+    }, &code, .Ignore) catch |err| switch (err) {
+        error.ExitCodeFailure => {}, // expected
+        else => return err,
+    };
+    const changes = code != 0;
+
+    return .{
+        .short_hash = short_hash,
+        .changes = changes,
+        .tag = if (tag.len > 0) std.mem.trimRight(u8, tag, "\r\n ") else null,
+        .branch = std.mem.trimRight(u8, branch, "\r\n "),
+    };
+}