fix build hangs

build.cm

@@ -91,6 +91,7 @@ var SALT_NATIVE = 'native'    // native-compiled .cm dylib
 var SALT_MACH = 'mach'        // mach bytecode blob
 var SALT_MCODE = 'mcode'      // mcode IR (JSON)
 var SALT_DEPS = 'deps'        // cached cc -MM dependency list
+var SALT_FAIL = 'fail'        // cached compilation failure
 
 function cache_path(content, salt) {
   return get_build_dir() + '/' + content_hash(content + '\n' + salt)
@@ -246,10 +247,18 @@ Build.compile_file = function(pkg, file, target, opts) {
   var file_content = fd.slurp(src_path)
   var quick_content = cmd_str + '\n' + text(file_content)
   var deps_path = cache_path(quick_content, SALT_DEPS)
+  var fail_path = cache_path(quick_content, SALT_FAIL)
 
   var build_dir = get_build_dir()
   ensure_dir(build_dir)
 
+  // Check for cached failure (skip files that previously failed to compile)
+  if (fd.is_file(fail_path)) {
+    if (_opts.verbose) print('[verbose] skipping ' + file + ' (cached failure)')
+    log.shop('skip ' + file + ' (cached failure)')
+    return null
+  }
+
   var deps = null
   var full_content = null
   var obj_path = null
@@ -310,6 +319,8 @@ Build.compile_file = function(pkg, file, target, opts) {
       if (err_text) print(err_text)
       else print('Command: ' + full_cmd)
     }
+    // Cache the failure so we don't retry on every build
+    fd.slurpwrite(fail_path, stone(blob(err_text || 'compilation failed')))
     return null
   }
 
@@ -944,6 +955,7 @@ Build.SALT_NATIVE = SALT_NATIVE
 Build.SALT_MACH = SALT_MACH
 Build.SALT_MCODE = SALT_MCODE
 Build.SALT_DEPS = SALT_DEPS
+Build.SALT_FAIL = SALT_FAIL
 Build.cache_path = cache_path
 
 return Build