v3: avoid Linux C header symbol conflicts

Author: medvednikov

vlib/v3/gen/c/cleanc.v

@@ -337,6 +337,7 @@ mut:
 	inlined_c_fns                  map[string]bool
 	inlined_c_declared_fns         map[string]bool
 	possibly_active_c_macros       map[string]bool
+	unscanned_c_header_files       map[string]bool
 	inlined_c_static_fns           map[string]bool
 	cache_omitted_c_fns            map[string]bool
 	preserved_header_files_seen    map[string]bool
@@ -1030,6 +1031,7 @@ pub fn FlatGen.new() FlatGen {
 		inlined_c_fns:                   map[string]bool{}
 		inlined_c_declared_fns:          map[string]bool{}
 		possibly_active_c_macros:        map[string]bool{}
+		unscanned_c_header_files:        map[string]bool{}
 		inlined_c_static_fns:            map[string]bool{}
 		cache_omitted_c_fns:             map[string]bool{}
 		preserved_header_files_seen:     map[string]bool{}
@@ -2616,6 +2618,7 @@ pub fn (mut g FlatGen) gen_with_used_options(a &flat.FlatAst, used_fns map[strin
 	g.inlined_c_fns.clear()
 	g.inlined_c_declared_fns.clear()
 	g.possibly_active_c_macros.clear()
+	g.unscanned_c_header_files.clear()
 	g.inlined_c_static_fns.clear()
 	g.cache_omitted_c_fns.clear()
 	g.preserved_header_files_seen.clear()
@@ -4421,6 +4424,10 @@ fn (mut g FlatGen) collect_c_directive(module_name string, node flat.Node, sourc
 			}
 		} else if c_should_preserve_uninlined_include(include_arg) || (g.cache_split
 			&& include_arg in ['<mach/mach.h>', '<mach/task.h>', '<mach/mach_time.h>']) {
+			// The preserved header is emitted before generated externs. When V cannot
+			// inspect it, avoid guessing prototypes that may differ in const qualifiers
+			// or other ABI details from the real declaration.
+			g.unscanned_c_header_files[source_file] = true
 			g.collect_preserved_c_fns(c_preserved_system_include_declared_fns(include_arg))
 			g.collect_preserved_c_structs(c_preserved_system_include_struct_names(include_arg))
 			g.collect_preserved_c_typedef_names(c_preserved_system_include_typedef_names(include_arg))

vlib/v3/gen/c/fn.v

@@ -1053,7 +1053,9 @@ const c_main_runtime_shadow_fn_names = {
 }
 
 fn (g &FlatGen) main_runtime_shadow_fn_c_name(module_name string, name string) ?string {
-	if !c_main_runtime_shadow_fn_names[name] && !g.inlined_c_typedef_names[name] {
+	c_type_name := !isnil(g.tc)
+		&& ('C.${name}' in g.tc.structs || 'C.${name}' in g.tc.c_typedef_structs)
+	if !c_main_runtime_shadow_fn_names[name] && !g.inlined_c_typedef_names[name] && !c_type_name {
 		return none
 	}
 	if module_name.len == 0 || module_name == 'main' {
@@ -15094,6 +15096,9 @@ fn (g &FlatGen) c_extern_decl_is_cached_object_fallback(cfn string) bool {
 }
 
 fn (g &FlatGen) should_emit_c_extern_decl_from_file(cfn string, source_file string) bool {
+	if g.unscanned_c_header_files[source_file] {
+		return false
+	}
 	// builtin/cfns.c.v declares the static vschannel helper supplied by its C header.
 	// A user C.request declaration is unrelated and still needs an extern prototype.
 	if cfn == 'request' && source_file.replace('\\', '/').ends_with('/builtin/cfns.c.v') {

vlib/v3/gen/c/names_test.v

@@ -111,6 +111,19 @@ fn test_main_function_is_prefixed_when_preserved_c_header_owns_typedef_name() {
 	assert g.fn_c_name_in_module('database', 'sqlite3') == 'database__sqlite3'
 }
 
+fn test_main_function_is_prefixed_when_declared_c_type_owns_name() {
+	mut a := flat.FlatAst.new()
+	mut tc := types.TypeChecker.new(&a)
+	mut g := FlatGen.new()
+	g.a = &a
+	g.tc = &tc
+	tc.structs['C.sqlite3'] = []types.StructField{}
+
+	assert g.fn_c_name_in_module('main', 'sqlite3') == 'main__sqlite3'
+	assert g.main_runtime_shadow_fn_c_name('main', 'sqlite3') or { '' } == 'main__sqlite3'
+	assert g.fn_c_name_in_module('database', 'sqlite3') == 'database__sqlite3'
+}
+
 fn test_voidptr_method_value_arg_does_not_panic_for_alias_to_voidptr() {
 	mut a := flat.FlatAst.new()
 	mut tc := types.TypeChecker.new(&a)

vlib/v3/gen/c/source_directive_test.v

@@ -77,6 +77,7 @@ fn test_postinclude_header_does_not_suppress_earlier_c_prototype() {
 	}, source, false)
 	assert 'postinclude_api' !in postinclude_g.inlined_c_declared_fns
 	assert '#include "${header}"' in postinclude_g.postinclude_directives
+	assert postinclude_g.should_emit_c_extern_decl_from_file('postinclude_api', source)
 
 	mut preinclude_g := FlatGen.new()
 	preinclude_g.collect_c_directive('main', flat.Node{
@@ -87,6 +88,23 @@ fn test_postinclude_header_does_not_suppress_earlier_c_prototype() {
 	assert 'postinclude_api' in preinclude_g.inlined_c_declared_fns
 }
 
+fn test_unscanned_preserved_header_suppresses_guessed_externs_from_its_source_file() {
+	root := os.join_path(os.vtmp_dir(), 'v3_unscanned_header_${os.getpid()}')
+	source := os.join_path(root, 'main.v')
+	missing_header := os.join_path(root, 'compiler-search-only', 'api.h')
+
+	mut g := FlatGen.new()
+	g.collect_c_directive('main', flat.Node{
+		kind:  .directive
+		value: 'include'
+		typ:   '"${missing_header}"'
+	}, source, false)
+
+	assert source in g.unscanned_c_header_files
+	assert !g.should_emit_c_extern_decl_from_file('header_api', source)
+	assert g.should_emit_c_extern_decl_from_file('header_api', os.join_path(root, 'other.v'))
+}
+
 fn test_preinclude_carries_macro_state_to_later_preincludes() {
 	root := os.join_path(os.vtmp_dir(), 'v3_preinclude_macro_state_${os.getpid()}')
 	os.rmdir_all(root) or {}