systemctl: add "edit --stdin"

This is a fancy wrapper around "cat <<EOF", but:
- the user doesn't need to figure out the file name,
- parent directories are created automatically,
- daemon-reload is implied,
so it's a convenient way to create units or drop-ins.

Closes https://github.com/systemd/systemd/issues/21862.

src/shared/edit-util.c

@@ -178,7 +178,7 @@ static int populate_edit_temp_file(EditFile *e, FILE *f, const char *filename) {
         return 0;
 }
 
-static int create_edit_temp_file(EditFile *e) {
+static int create_edit_temp_file(EditFile *e, const char *contents, size_t contents_size) {
         _cleanup_(unlink_and_freep) char *temp = NULL;
         _cleanup_fclose_ FILE *f = NULL;
         int r;
@@ -187,6 +187,7 @@ static int create_edit_temp_file(EditFile *e) {
         assert(e->context);
         assert(e->path);
         assert(!e->comment_paths || (e->context->marker_start && e->context->marker_end));
+        assert(contents || contents_size == 0);
 
         if (e->temp)
                 return 0;
@@ -202,9 +203,15 @@ static int create_edit_temp_file(EditFile *e) {
         if (fchmod(fileno(f), 0644) < 0)
                 return log_error_errno(errno, "Failed to change mode of temporary file '%s': %m", temp);
 
-        r = populate_edit_temp_file(e, f, temp);
-        if (r < 0)
-                return r;
+        if (e->context->stdin) {
+                if (fwrite(contents, 1, contents_size, f) != contents_size)
+                        return log_error_errno(SYNTHETIC_ERRNO(EIO),
+                                               "Failed to copy input to temporary file '%s': %m", temp);
+        } else {
+                r = populate_edit_temp_file(e, f, temp);
+                if (r < 0)
+                        return r;
+        }
 
         r = fflush_and_check(f);
         if (r < 0)
@@ -310,7 +317,7 @@ static int strip_edit_temp_file(EditFile *e) {
         if (!tmp)
                 return log_oom();
 
-        if (e->context->marker_start) {
+        if (e->context->marker_start && !e->context->stdin) {
                 /* Trim out the lines between the two markers */
                 char *contents_start, *contents_end;
 
@@ -349,6 +356,8 @@ static int strip_edit_temp_file(EditFile *e) {
 }
 
 int do_edit_files_and_install(EditFileContext *context) {
+        _cleanup_free_ char *data = NULL;
+        size_t data_size = 0;
         int r;
 
         assert(context);
@@ -356,33 +365,41 @@ int do_edit_files_and_install(EditFileContext *context) {
         if (context->n_files == 0)
                 return log_debug_errno(SYNTHETIC_ERRNO(ENOENT), "Got no files to edit.");
 
-        FOREACH_ARRAY(i, context->files, context->n_files) {
-                r = create_edit_temp_file(i);
+        if (context->stdin) {
+                r = read_full_stream(stdin, &data, &data_size);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to read stdin: %m");
+        }
+
+        FOREACH_ARRAY(editfile, context->files, context->n_files) {
+                r = create_edit_temp_file(editfile, data, data_size);
                 if (r < 0)
                         return r;
         }
 
-        r = run_editor(context);
-        if (r < 0)
-                return r;
+        if (!context->stdin) {
+                r = run_editor(context);
+                if (r < 0)
+                        return r;
+        }
 
-        FOREACH_ARRAY(i, context->files, context->n_files) {
+        FOREACH_ARRAY(editfile, context->files, context->n_files) {
                 /* Always call strip_edit_temp_file which will tell if the temp file has actual changes */
-                r = strip_edit_temp_file(i);
+                r = strip_edit_temp_file(editfile);
                 if (r < 0)
                         return r;
                 if (r == 0) /* temp file doesn't carry actual changes, ignoring */
                         continue;
 
-                r = RET_NERRNO(rename(i->temp, i->path));
+                r = RET_NERRNO(rename(editfile->temp, editfile->path));
                 if (r < 0)
                         return log_error_errno(r,
                                                "Failed to rename temporary file '%s' to target file '%s': %m",
-                                               i->temp,
-                                               i->path);
-                i->temp = mfree(i->temp);
+                                               editfile->temp,
+                                               editfile->path);
+                editfile->temp = mfree(editfile->temp);
 
-                log_info("Successfully installed edited file '%s'.", i->path);
+                log_info("Successfully installed edited file '%s'.", editfile->path);
         }
 
         return 0;

src/shared/edit-util.h

@@ -24,7 +24,8 @@ struct EditFileContext {
         const char *marker_start;
         const char *marker_end;
         bool remove_parent;
-        bool overwrite_with_origin; /* whether to always overwrite target with original file */
+        bool overwrite_with_origin; /* Always overwrite target with original file. */
+        bool stdin;                 /* Read contents from stdin instead of launching an editor. */
 };
 
 void edit_file_context_done(EditFileContext *context);