Reinstate the use of $TMPDIR and $TMP in cursor positioning rather than hard-coding "/tmp" (#88).

docs/NEWS.md

@@ -1,6 +1,7 @@
 ### NOT YET RELEASED
 
  * feature: new "`--si`" option to display and interpret size suffixes in multiples of 1000 rather than 1024 (pull request [#85](https://codeberg.org/a-j-wood/pv/pulls/85)) supplied by [kevinruddy](https://codeberg.org/kevinruddy)
+ * fix: honour the _TMPDIR_ / _TMP_ environment variables again, rather than hard-coding "`/tmp`", when using a terminal lock file (originally removed in 1.8.0) ([#88](https://codeberg.org/a-j-wood/pv/issues/88))
  * i18n: corrections and missing strings added to French translations (pull request [#83](https://codeberg.org/a-j-wood/pv/pulls/83)) supplied by [Thomas Bertels](https://codeberg.org/tbertels)
 
 ### 1.8.5 - 19 November 2023

src/pv/cursor.c

@@ -95,14 +95,16 @@ static void pv_crs_open_lockfile(pvstate_t state, int fd)
 		return;
 	}
 
+	tmpdir = (char *) getenv("TMPDIR");		/* flawfinder: ignore */
+	if ((NULL == tmpdir) || ('\0' == tmpdir[0]))
+		tmpdir = (char *) getenv("TMP");	/* flawfinder: ignore */
+	if ((NULL == tmpdir) || ('\0' == tmpdir[0]))
+		tmpdir = "/tmp";
+
 	/*
-	 * We used to look at the TMPDIR or TMP environment variables to
-	 * override the temporary directory, but this leads to less
-	 * predictable behaviour, and flawfinder points out that relying on
-	 * environment variables in this way is not safe.  So the lock
-	 * directory is hard-coded to "/tmp" now (Sep 2023).
+	 * flawfinder rationale: null and zero-size values of $TMPDIR and
+	 * $TMP are rejected, and the destination buffer is bounded.
 	 */
-	tmpdir = "/tmp";
 
 	memset(state->cursor.lock_file, 0, PV_SIZEOF_CRS_LOCK_FILE);
 	(void) pv_snprintf(state->cursor.lock_file,