lib: improve i_close_fd() error message

In addition to printing the file and line number, we can print the
actual arg passed in as well as the function calling i_close_fd().

src/lib/lib.c

@@ -41,7 +41,8 @@ int close_keep_errno(int *fd)
 	return ret;
 }
 
-void i_close_fd_real(int *fd, const char *file, int line)
+void i_close_fd_real(int *fd, const char *arg, const char *func,
+		     const char *file, int line)
 {
 	int saved_errno;
 
@@ -52,7 +53,8 @@ void i_close_fd_real(int *fd, const char *file, int line)
 
 	saved_errno = errno;
 	if (unlikely(close(*fd) < 0))
-		i_error("close(%d[%s:%d]) failed: %m", *fd, file, line);
+		i_error("%s: close(%s) @ %s:%d failed (fd=%d): %m",
+			func, arg, file, line, *fd);
 	errno = saved_errno;
 
 	*fd = -1;

src/lib/macros.h

@@ -212,8 +212,8 @@
 /* Close the fd and set it to -1. This assert-crashes if fd == 0, and is a
    no-op if fd == -1. Normally fd == 0 would happen only if an uninitialized
    fd is attempted to be closed, which is a bug. */
-void i_close_fd_real(int *fd, const char *file, int line);
-#define i_close_fd(fd) i_close_fd_real((fd), __FILE__, __LINE__)
+void i_close_fd_real(int *fd, const char *arg, const char *func, const char *file, int line);
+#define i_close_fd(fd) i_close_fd_real((fd), #fd, __func__, __FILE__, __LINE__)
 
 #ifndef STATIC_CHECKER
 #  define i_unreached() \