generator: add helper function for writing unit files

It doesn't save too much, but it's a common pattern so I think it's worth
to factor this out.

src/shared/generator.c

@@ -19,6 +19,7 @@
 ***/
 
 #include <errno.h>
+#include <stdio_ext.h>
 #include <unistd.h>
 
 #include "alloc-util.h"
@@ -39,6 +40,39 @@
 #include "unit-name.h"
 #include "util.h"
 
+int generator_open_unit_file(
+                const char *dest,
+                const char *source,
+                const char *name,
+                FILE **file) {
+
+        const char *unit;
+        FILE *f;
+
+        unit = strjoina(dest, "/", name);
+
+        f = fopen(unit, "wxe");
+        if (!f) {
+                if (source && errno == EEXIST)
+                        return log_error_errno(errno,
+                                               "Failed to create unit file %s, as it already exists. Duplicate entry in %s?",
+                                               unit, source);
+                else
+                        return log_error_errno(errno,
+                                               "Failed to create unit file %s: %m",
+                                               unit);
+        }
+
+        (void) __fsetlocking(f, FSETLOCKING_BYCALLER);
+
+        fprintf(f,
+                "# Automatically generated by %s\n\n",
+                program_invocation_short_name);
+
+        *file = f;
+        return 0;
+}
+
 int generator_add_symlink(const char *root, const char *dst, const char *dep_type, const char *src) {
         /* Adds a symlink from <dst>.<dep_type>.d/ to ../<src> */
 

src/shared/generator.h

@@ -22,6 +22,12 @@
 
 #include <stdio.h>
 
+int generator_open_unit_file(
+        const char *dest,
+        const char *source,
+        const char *name,
+        FILE **file);
+
 int generator_add_symlink(const char *root, const char *dst, const char *dep_type, const char *src);
 
 int generator_write_fsck_deps(