solve problem of finding paths for anonymous modules

src/universal_lambda_build.zig

@@ -20,8 +20,10 @@ pub fn configureBuild(b: *std.Build, exe: *std.Build.Step.Compile) !void {
     // type of interface is necessary
 
     // Add module
+    const file_location = try findFileLocation(b);
     exe.addAnonymousModule("universal_lambda_handler", .{
-        .source_file = .{ .path = "upstream/src/universal_lambda.zig" },
+        // Source file can be anywhere on disk, does not need to be a subdirectory
+        .source_file = .{ .path = b.pathJoin(&[_][]const u8{ file_location, "universal_lambda.zig" }) },
         .dependencies = &[_]std.Build.ModuleDependency{.{
             .name = "build_options",
             .module = try createOptionsModule(b, exe),
@@ -29,6 +31,41 @@ pub fn configureBuild(b: *std.Build, exe: *std.Build.Step.Compile) !void {
     });
 }
 
+/// This function relies on internal implementation of the build runner
+/// When a developer launches "zig build", a program is compiled, with the
+/// main entrypoint existing in build_runner.zig (this can be overridden by
+/// by command line).
+///
+/// The code we see in build.zig is compiled into that program. The program
+/// is named 'build' and stuck in the zig cache, then it is run. There are
+/// two phases to the build.
+///
+/// Build phase, where a graph is established of the steps that need to be run,
+/// then the "make phase", where the steps are actually executed. Steps have
+/// a make function that is called.
+///
+/// This function is reaching into the struct that is the build_runner.zig, and
+/// finding the location of the dependency for ourselves to determine the
+/// location of our own file. This is, of course, brittle, but there does not
+/// seem to be a better way at the moment, and we need this to be able to import
+/// modules appropriately.
+///
+/// For development of this process directly, we'll allow a build option to
+/// override this process completely, because during development it's easier
+/// for the example build.zig to simply import the file directly than it is
+/// to pull from a download location and update hashes every time we change
+fn findFileLocation(b: *std.Build) ![]const u8 {
+    const build_root = b.option([]const u8, "universal_lambda_build_root", "Build root for universal lambda (development of universal lambda only)");
+    if (build_root) |br| {
+        return b.pathJoin(&[_][]const u8{ br, "src" });
+    }
+    const build_runner = @import("root");
+    const deps = build_runner.dependencies;
+    const build_roots = deps.build_root;
+    if (!@hasField(build_roots, "universal_lambda_build"))
+        @panic("Dependency in build.zig.zon must be named 'universal_lambda_build'");
+    return b.pathJoin(&[_][]const u8{ @field(build_roots, "universal_lambda_build"), "src" });
+}
 /// Make our target platform visible to runtime through an import
 /// called "build_options". This will also be available to the consuming
 /// executable if needed