build: add semantic versioning support

both programs now acquire semantic versioning recorded at the build
time. they also report the version at startup and -v flag.
this is useful for a more formal release process and to potentially
avoid compatibility issues in the future.

in a regular build flow, the version is taken from a git tag using the
following command:

    git -C . describe --match 'v*.*.*' --tags

in a non-standard scenario where git isn't available, the version can
be provided on the command line during build like so:

    zig build -Dversion=1.2.3

if both git and command line supplied versions are available, they must
match.

build.zig

@@ -10,9 +10,11 @@ pub fn build(b: *std.build.Builder) void {
     const disp_horiz = b.option(u32, "horiz", "display horizontal pixels count; default: 800") orelse 800;
     const disp_vert = b.option(u32, "vert", "display vertical pixels count; default: 480") orelse 480;
     const lvgl_loglevel = b.option(LVGLLogLevel, "lvgl_loglevel", "LVGL lib logging level") orelse LVGLLogLevel.default(mode);
+    const inver = b.option([]const u8, "version", "semantic version of the build; must match git tag when available");
 
     const buildopts = b.addOptions();
     buildopts.addOption(DriverTarget, "driver", drv);
+    const semver_step = VersionStep.create(b, buildopts, inver);
 
     // gui build
     const ngui = b.addExecutable("ngui", "src/ngui.zig");
@@ -20,6 +22,7 @@ pub fn build(b: *std.build.Builder) void {
     ngui.setBuildMode(mode);
     ngui.pie = true;
     ngui.strip = strip;
+    ngui.step.dependOn(semver_step);
 
     ngui.addPackage(buildopts.getPackage("build_options"));
     ngui.addIncludePath("lib");
@@ -86,6 +89,7 @@ pub fn build(b: *std.build.Builder) void {
     nd.setBuildMode(mode);
     nd.pie = true;
     nd.strip = strip;
+    nd.step.dependOn(semver_step);
 
     nd.addPackage(buildopts.getPackage("build_options"));
     nifbuild.addPkg(b, nd, "lib/nif");
@@ -314,3 +318,75 @@ const LVGLLogLevel = enum {
         };
     }
 };
+
+/// VersionStep injects a release build semantic version into buildopts as "semver".
+/// the make step fails if the inver input version and the one found in a git tag mismatch.
+///
+/// while git-tagged versions are expected to be in v<semver>format, input version
+/// to match against is any format supported by std.SemanticVersion.parse.
+/// input version is optional; if unset, make fn succeeds given a correctly formatted
+/// git tag is found.
+const VersionStep = struct {
+    inver: ?[]const u8, // input version in std.SemanticVersion.parse format
+    buildopts: *std.build.OptionsStep, // where to store the build version
+
+    b: *std.build.Builder,
+    step: std.build.Step,
+
+    fn create(b: *std.build.Builder, o: *std.build.OptionsStep, inver: ?[]const u8) *std.build.Step {
+        const vstep = b.allocator.create(VersionStep) catch unreachable;
+        vstep.* = VersionStep{
+            .inver = inver,
+            .buildopts = o,
+            .b = b,
+            .step = std.build.Step.init(.custom, "VersionStep: ndg semver", b.allocator, make),
+        };
+        return &vstep.step;
+    }
+
+    fn make(step: *std.build.Step) anyerror!void {
+        const self = @fieldParentPtr(VersionStep, "step", step);
+        const semver = try self.eval();
+        std.log.info("build version: {any}", .{semver});
+        self.buildopts.addOption(std.SemanticVersion, "semver", semver);
+    }
+
+    fn eval(self: *VersionStep) !std.SemanticVersion {
+        const repover = try self.gitver();
+        if (self.inver) |v| {
+            const insem = std.SemanticVersion.parse(v) catch |err| {
+                std.log.err("invalid input semver '{s}': {any}", .{ v, err });
+                return err;
+            };
+            if (repover != null and insem.order(repover.?) != .eq) {
+                std.log.err("input and repo semver mismatch: {any} vs {any}", .{ insem, repover });
+                return error.VersionMismatch;
+            }
+            return insem;
+        }
+
+        if (repover == null) {
+            std.log.err("must supply build semver from command line.", .{});
+            return error.MissingVersion;
+        }
+        return repover.?;
+    }
+
+    fn gitver(self: *VersionStep) !?std.SemanticVersion {
+        if (!std.process.can_spawn) {
+            return null;
+        }
+        const git = self.b.findProgram(&[_][]const u8{"git"}, &[_][]const u8{}) catch return null;
+
+        const prefix = "v"; // git tag prefix
+        const matchTag = self.b.fmt("{s}*.*.*", .{prefix});
+        const cmd = [_][]const u8{ git, "-C", self.b.pathFromRoot("."), "describe", "--match", matchTag, "--tags" };
+        var code: u8 = undefined;
+        const git_describe = self.b.execAllowFail(&cmd, &code, .Ignore) catch return null;
+        const repotag = std.mem.trim(u8, git_describe, " \n\r")[prefix.len..];
+        return std.SemanticVersion.parse(repotag) catch |err| ret: {
+            std.log.err("unparsable git tag semver '{s}': {any}", .{ repotag, err });
+            break :ret err;
+        };
+    }
+};

src/nd.zig

@@ -1,3 +1,4 @@
+const buildopts = @import("build_options");
 const std = @import("std");
 const os = std.os;
 const sys = os.system;
@@ -86,6 +87,9 @@ fn parseArgs(gpa: std.mem.Allocator) !NdArgs {
         if (std.mem.eql(u8, a, "-h") or std.mem.eql(u8, a, "-help") or std.mem.eql(u8, a, "--help")) {
             usage(prog) catch {};
             std.process.exit(1);
+        } else if (std.mem.eql(u8, a, "-v")) {
+            try stderr.print("{any}\n", .{buildopts.nd_semver});
+            std.process.exit(0);
         } else if (std.mem.eql(u8, a, "-gui")) {
             lastarg = .gui;
         } else if (std.mem.eql(u8, a, "-gui-user")) {
@@ -126,6 +130,7 @@ pub fn main() !void {
     // parse program args first thing and fail fast if invalid
     const args = try parseArgs(gpa);
     defer args.deinit(gpa);
+    logger.info("version {any}", .{buildopts.nd_semver});
 
     // reset the screen backlight to normal power regardless
     // of its previous state.

src/ngui.zig

@@ -1,3 +1,4 @@
+const buildopts = @import("build_options");
 const std = @import("std");
 const time = std.time;
 
@@ -16,6 +17,7 @@ pub const keep_sigpipe = true;
 
 const stdin = std.io.getStdIn().reader();
 const stdout = std.io.getStdOut().writer();
+const stderr = std.io.getStdErr().writer();
 const logger = std.log.scoped(.ngui);
 
 extern "c" fn ui_update_network_status(text: [*:0]const u8, wifi_list: ?[*:0]const u8) void;
@@ -177,18 +179,59 @@ fn commThreadLoopCycle() !void {
     }
 }
 
+/// prints messages in the same way std.fmt.format does and exits the process
+/// with a non-zero code.
+fn fatal(comptime fmt: []const u8, args: anytype) noreturn {
+    stderr.print(fmt, args) catch {};
+    if (fmt[fmt.len - 1] != '\n') {
+        stderr.writeByte('\n') catch {};
+    }
+    std.process.exit(1);
+}
+
+fn parseArgs(alloc: std.mem.Allocator) !void {
+    var args = try std.process.ArgIterator.initWithAllocator(alloc);
+    defer args.deinit();
+    const prog = args.next() orelse return error.NoProgName;
+
+    while (args.next()) |a| {
+        if (std.mem.eql(u8, a, "-h") or std.mem.eql(u8, a, "-help") or std.mem.eql(u8, a, "--help")) {
+            usage(prog) catch {};
+            std.process.exit(1);
+        } else if (std.mem.eql(u8, a, "-v")) {
+            try stderr.print("{any}\n", .{buildopts.ngui_semver});
+            std.process.exit(0);
+        } else {
+            fatal("unknown arg name {s}", .{a});
+        }
+    }
+}
+
+/// prints usage help text to stderr.
+fn usage(prog: []const u8) !void {
+    try stderr.print(
+        \\usage: {s} [-v]
+        \\
+        \\ngui is nakamochi GUI interface. it communicates with nd, nakamochi daemon,
+        \\via stdio and is typically launched by the daemon as a child process.
+        \\
+    , .{prog});
+}
+
 /// nakamochi UI program entry point.
 pub fn main() anyerror!void {
-    // ensure timer is available on this platform before doing anything else;
-    // the UI is unusable otherwise.
-    tick_timer = try time.Timer.start();
-
     // main heap allocator used through the lifetime of nd
     var gpa_state = std.heap.GeneralPurposeAllocator(.{}){};
     defer if (gpa_state.deinit()) {
         logger.err("memory leaks detected", .{});
     };
     gpa = gpa_state.allocator();
+    try parseArgs(gpa);
+    logger.info("version {any}", .{buildopts.ngui_semver});
+
+    // ensure timer is available on this platform before doing anything else;
+    // the UI is unusable otherwise.
+    tick_timer = try time.Timer.start();
 
     // initalizes display, input driver and finally creates the user interface.
     ui.init() catch |err| {