clang-tidy: add to CI, add cmake support, fix fallouts

build:
- autotools: fix to build generated sources for the `tidy` target.
- autotools: allow passing custom clang-tidy options via
  `CURL_CLANG_TIDYFLAGS` env.
- cmake: add `CURL_CLANG_TIDY` option to configure for `clang-tidy`.
  Also add:
  - `CLANG_TIDY` variable to customize the `clang-tidy` tool.
  - `CURL_CLANG_TIDYFLAGS` to pass custom options to `clang-tidy`.
- apply `--enable-werror` and `-DCURL_WERROR=ON` to `clang-tidy`.

CI/GHA:
- add clang-tidy job for Linux, using autotools and clang-tidy v18.
  This one needs to disable `clang-analyzer-valist.Uninitialized`
  to avoid false positives:
  https://github.com/llvm/llvm-project/issues/40656
  Duration: 5.5 minutes
- add clang-tidy job for macOS, using cmake and clang-tidy v19.
  This one also covers tests and examples, and doesn't hit the false
  positives seen with llvm v18 and earlier.
  Duration: 4.5 minutes
- Linux/macOS: skip installing test dependencies when not building or
  running tests.

fix fallouts reported by `clang-tidy`:
- lib:
  - cf-h2-proxy: unused assignment in non-debug builds.
  - cf-socket: silence warning.
    FIXME: https://github.com/curl/curl/pull/15825#issuecomment-2561867769
  - ftp: NULL passed to `strncmp()`.
  - http2: NULL-ptr deref.
  - mprintf: silence warning.
- src/tool_writeout: NULL passed to `fputs()`.
- examples:
  - invalid file pointers.
  - missing `fclose()`.
- tests:
  - http/clients/hx-download: memory leaks on error.
  - http/clients/hx-download: memory leak on repeat `-r` option.
  - server: double `fclose()`.
    https://www.man7.org/linux/man-pages/man3/fclose.3.html
  - server: invalid file pointer/handle.
  - server/getpart: unused assignments.
  - server/mqttd: leak on failed `realloc()`.
  - server/tftpd: NULL passed to `strcmp()`.

Closes #15825

tests/server/getpart.c

@@ -349,9 +349,6 @@ int getpart(char **outbuf, size_t *outlen,
         state = STATE_INMAIN;
         csub[0] = '\0';
         if(in_wanted_part) {
-          /* end of wanted part */
-          in_wanted_part = 0;
-
           /* Do we need to base64 decode the data? */
           if(base64) {
             error = decodedata(outbuf, outlen);
@@ -368,9 +365,6 @@ int getpart(char **outbuf, size_t *outlen,
         state = STATE_OUTER;
         cmain[0] = '\0';
         if(in_wanted_part) {
-          /* end of wanted part */
-          in_wanted_part = 0;
-
           /* Do we need to base64 decode the data? */
           if(base64) {
             error = decodedata(outbuf, outlen);
@@ -386,11 +380,8 @@ int getpart(char **outbuf, size_t *outlen,
         /* end of outermost file section */
         state = STATE_OUTSIDE;
         couter[0] = '\0';
-        if(in_wanted_part) {
-          /* end of wanted part */
-          in_wanted_part = 0;
+        if(in_wanted_part)
           break;
-        }
       }
 
     }

tests/server/mqttd.c

@@ -542,12 +542,14 @@ static curl_socket_t mqttit(curl_socket_t fd)
       break;
 
     if(remaining_length >= buff_size) {
+      unsigned char *newbuffer;
       buff_size = remaining_length;
-      buffer = realloc(buffer, buff_size);
-      if(!buffer) {
+      newbuffer = realloc(buffer, buff_size);
+      if(!newbuffer) {
         logmsg("Failed realloc of size %zu", buff_size);
         goto end;
       }
+      buffer = newbuffer;
     }
 
     if(remaining_length) {

tests/server/rtspd.c

@@ -641,12 +641,10 @@ static void storerequest(char *reqbuf, size_t totalsize)
 
 storerequest_cleanup:
 
-  do {
-    res = fclose(dump);
-  } while(res && ((error = errno) == EINTR));
+  res = fclose(dump);
   if(res)
     logmsg("Error closing file %s error: %d %s",
-           dumpfile, error, strerror(error));
+           dumpfile, errno, strerror(errno));
 }
 
 /* return 0 on success, non-zero on failure */
@@ -976,12 +974,10 @@ static int send_doc(curl_socket_t sock, struct httprequest *req)
     req->rtp_buffersize = 0;
   }
 
-  do {
-    res = fclose(dump);
-  } while(res && ((error = errno) == EINTR));
+  res = fclose(dump);
   if(res)
     logmsg("Error closing file %s error: %d %s",
-           responsedump, error, strerror(error));
+           responsedump, errno, strerror(errno));
 
   if(got_exit_signal) {
     free(ptr);

tests/server/socksd.c

@@ -1167,7 +1167,7 @@ socks5_cleanup:
     sclose(sock);
 
 #ifdef USE_UNIX_SOCKETS
-  if(unlink_socket && socket_domain == AF_UNIX) {
+  if(unlink_socket && socket_domain == AF_UNIX && unix_socket) {
     error = unlink(unix_socket);
     logmsg("unlink(%s) = %d (%s)", unix_socket, error, strerror(error));
   }

tests/server/sws.c

@@ -832,12 +832,10 @@ static void storerequest(const char *reqbuf, size_t totalsize)
 
 storerequest_cleanup:
 
-  do {
-    res = fclose(dump);
-  } while(res && ((error = errno) == EINTR));
+  res = fclose(dump);
   if(res)
     logmsg("Error closing file %s error: %d %s",
-           dumpfile, error, strerror(error));
+           dumpfile, errno, strerror(errno));
 }
 
 static void init_httprequest(struct httprequest *req)
@@ -1217,12 +1215,10 @@ retry:
     }
   } while((count > 0) && !got_exit_signal);
 
-  do {
-    res = fclose(dump);
-  } while(res && ((error = errno) == EINTR));
+  res = fclose(dump);
   if(res)
     logmsg("Error closing file %s error: %d %s",
-           responsedump, error, strerror(error));
+           responsedump, errno, strerror(errno));
 
   if(got_exit_signal) {
     free(ptr);
@@ -2527,7 +2523,7 @@ sws_cleanup:
     sclose(sock);
 
 #ifdef USE_UNIX_SOCKETS
-  if(unlink_socket && socket_domain == AF_UNIX) {
+  if(unlink_socket && socket_domain == AF_UNIX && unix_socket) {
     rc = unlink(unix_socket);
     logmsg("unlink(%s) = %d (%s)", unix_socket, rc, strerror(rc));
   }

tests/server/tftpd.c

@@ -425,9 +425,9 @@ static int writeit(struct testcase *test, struct tftphdr * volatile *dpp,
   bfs[current].counter = ct;      /* set size of data to write */
   current = !current;             /* switch to other buffer */
   if(bfs[current].counter != BF_FREE)     /* if not free */
-    write_behind(test, convert);     /* flush it */
+    write_behind(test, convert);          /* flush it */
   bfs[current].counter = BF_ALLOC;        /* mark as alloc'd */
-  *dpp =  &bfs[current].buf.hdr;
+  *dpp = &bfs[current].buf.hdr;
   return ct;                      /* this is a lie of course */
 }
 
@@ -448,7 +448,7 @@ static ssize_t write_behind(struct testcase *test, int convert)
   struct tftphdr *dp;
 
   b = &bfs[nextone];
-  if(b->counter < -1)            /* anything to flush? */
+  if(b->counter < -1)             /* anything to flush? */
     return 0;                     /* just nop if nothing to do */
 
   if(!test->ofile) {
@@ -464,6 +464,9 @@ static ssize_t write_behind(struct testcase *test, int convert)
       return -1; /* failure! */
     }
   }
+  else if(test->ofile <= 0) {
+    return -1; /* failure! */
+  }
 
   count = b->counter;             /* remember byte count */
   b->counter = BF_FREE;           /* reset flag */
@@ -952,7 +955,7 @@ static int do_tftp(struct testcase *test, struct tftphdr *tp, ssize_t size)
       break;
   } while(1);
 
-  if(*cp) {
+  if(*cp || !mode) {
     nak(EBADOP);
     fclose(server);
     return 3;

tests/server/util.c

@@ -341,12 +341,10 @@ void set_advisor_read_lock(const char *filename)
     return;
   }
 
-  do {
-    res = fclose(lockfile);
-  } while(res && ((error = errno) == EINTR));
+  res = fclose(lockfile);
   if(res)
     logmsg("Error closing lock file %s error: %d %s",
-           filename, error, strerror(error));
+           filename, errno, strerror(errno));
 }
 
 void clear_advisor_read_lock(const char *filename)