gen/c: resolve each file at most once in the native-input cache scan

vlib/v/gen/c/cache_program_file_test.v

@@ -0,0 +1,40 @@
+module c
+
+import os
+
+// cache_program_file_matches must accept a program file under any spelling that
+// resolves to it, and resolve each written path at most once per memo.
+fn test_cache_program_file_matches_resolves_each_written_path_once() {
+	root := os.join_path(os.temp_dir(), 'v_cache_program_file_${os.getpid()}')
+	os.rmdir_all(root) or {}
+	os.mkdir_all(root)!
+	defer {
+		os.rmdir_all(root) or {}
+	}
+	program := os.join_path(root, 'main.v')
+	other := os.join_path(root, 'other.v')
+	os.write_file(program, 'module main\n')!
+	os.write_file(other, 'module main\n')!
+	program_files := {
+		os.real_path(program): true
+	}
+	// This spelling only matches after resolution (join_path would drop the `.`).
+	dotted := root + os.path_separator + '.' + os.path_separator + 'main.v'
+	assert !program_files[dotted]
+	mut memo := map[string]bool{}
+	assert cache_program_file_matches(program_files, dotted, mut memo)
+	assert !cache_program_file_matches(program_files, other, mut memo)
+	assert memo == {
+		dotted: true
+		other:  false
+	}
+	// A memoized answer is used as is; the path is not resolved again.
+	mut seeded := {
+		other: true
+	}
+	assert cache_program_file_matches(program_files, other, mut seeded)
+	// Without program files nothing matches, and nothing is resolved or memoized.
+	mut empty_memo := map[string]bool{}
+	assert !cache_program_file_matches(map[string]bool{}, other, mut empty_memo)
+	assert empty_memo.len == 0
+}

vlib/v/gen/c/cleanc.v

@@ -1926,11 +1926,13 @@ pub fn cache_external_input_snapshot_with_resolved_flags(a &flat.FlatAst, vroot
 	mut preinclude_context_directives := []string{}
 	mut conditional_context_mutations := map[string]bool{}
 	mut conditionals := []CCacheConditional{}
+	mut program_file_memo := map[string]bool{}
 	for node_id in c_cache_external_input_node_order(a) {
 		node := a.nodes[node_id]
 		if node.kind == .file {
 			cur_file = node.value
-			cur_file_is_program = program_files[cur_file] || program_files[os.real_path(cur_file)]
+			cur_file_is_program = cache_program_file_matches(program_files, cur_file, mut
+				program_file_memo)
 			cur_module = ''
 			conditionals.clear()
 			continue

vlib/v/gen/c/fn.v

@@ -544,13 +544,19 @@ fn (mut g FlatGen) gen_fn_items(items []FlatFnGenItem) {
 // file_is_cache_program_file reports whether `file`, as written or resolved,
 // is one of the cached program files, memoizing the answer per file.
 fn (g &FlatGen) file_is_cache_program_file(file string, mut memo map[string]bool) bool {
-	if g.cache_program_files.len == 0 {
+	return cache_program_file_matches(g.cache_program_files, file, mut memo)
+}
+
+// cache_program_file_matches reports whether `file`, as written or resolved, is
+// one of `program_files`. It resolves each written path at most once per memo.
+fn cache_program_file_matches(program_files map[string]bool, file string, mut memo map[string]bool) bool {
+	if program_files.len == 0 {
 		return false
 	}
 	if known := memo[file] {
 		return known
 	}
-	is_program := g.cache_program_files[file] || g.cache_program_files[os.real_path(file)]
+	is_program := program_files[file] || program_files[os.real_path(file)]
 	memo[file] = is_program
 	return is_program
 }