builder: add a verror, when the output folder is not existing and writable (#5966)

vlib/os/os.v

@@ -511,23 +511,6 @@ pub fn is_executable(path string) bool {
   return C.access(charptr(path.str), x_ok) != -1
 }
 
-// `is_writable_folder` - `folder` exists and is writable to the process
-pub fn is_writable_folder(folder string) ?bool {
-	if !os.exists(folder) {
-		return error('`$folder` does not exist')
-	}
-	if !os.is_dir(folder) {
-		return error('`folder` is not a folder')
-	}
-	tmp_perm_check := os.join_path(folder, 'tmp_perm_check')
-	mut f := os.open_file(tmp_perm_check, 'w+', 0o700) or {
-		return error('cannot write to folder `$folder`: $err')
-	}
-	f.close()
-	os.rm(tmp_perm_check)
-	return true
-}
-
 // `is_writable` returns `true` if `path` is writable.
 pub fn is_writable(path string) bool {
   $if windows {
@@ -638,7 +621,7 @@ pub fn dir(path string) string {
 
 pub fn base_dir(path string) string {
 	posx := path.last_index(path_separator) or {
-		return path
+		return path.clone()
 	}
 	// NB: *without* terminating /
 	return path[..posx]

vlib/os/os_nix.c.v

@@ -190,3 +190,24 @@ pub fn (mut f File) close() {
 pub fn debugger_present() bool {
 	return false
 }
+
+fn C.mkstemp(stemplate byteptr) int
+// `is_writable_folder` - `folder` exists and is writable to the process
+pub fn is_writable_folder(folder string) ?bool {
+	if !os.exists(folder) {
+		return error('`$folder` does not exist')
+	}
+	if !os.is_dir(folder) {
+		return error('`folder` is not a folder')
+	}
+	tmp_perm_check := os.join_path(folder, 'XXXXXX')
+	unsafe {
+		x := C.mkstemp(tmp_perm_check.str)
+		if -1 == x {
+			return error('folder `$folder` is not writable')
+		}
+		C.close(x)
+	}
+	os.rm(tmp_perm_check)
+	return true
+}

vlib/os/os_windows.c.v

@@ -366,3 +366,21 @@ pub fn uname() Uname {
 		machine: unknown
 	}
 }
+
+
+// `is_writable_folder` - `folder` exists and is writable to the process
+pub fn is_writable_folder(folder string) ?bool {
+	if !os.exists(folder) {
+		return error('`$folder` does not exist')
+	}
+	if !os.is_dir(folder) {
+		return error('`folder` is not a folder')
+	}
+	tmp_perm_check := os.join_path(folder, 'tmp_perm_check')
+	mut f := os.open_file(tmp_perm_check, 'w+', 0o700) or {
+		return error('cannot write to folder $folder: $err')
+	}
+	f.close()
+	os.rm(tmp_perm_check)
+	return true
+}

vlib/v/builder/compile.v

@@ -27,6 +27,18 @@ fn get_vtmp_filename(base_file_name, postfix string) string {
 }
 
 pub fn compile(command string, pref &pref.Preferences) {
+	odir := os.base_dir(pref.out_name)
+	// When pref.out_name is just the name of an executable, i.e. `./v -o executable main.v`
+	// without a folder component, just use the current folder instead:
+	mut output_folder := odir
+	if odir.len == pref.out_name.len {
+		output_folder = os.getwd()
+	}        
+	os.is_writable_folder(output_folder) or { 
+		// An early error here, is better than an unclear C error later:
+		verror(err)
+		exit(1)
+	}
 	// Construct the V object from command line arguments
 	mut b := new_builder(pref)
 	if pref.is_verbose {