require explicit check for pixel support

Add `notcurses_check_pixel_support()` and
`ncdirect_check_pixel_support()` per #1367. Removes
NCOPTION_VERIFY_SIXEL, again per #1367. Adds
`free_terminfo_cache()`, and calls it from both
`notcurses_stop_minimal()` and `ncdirect_stop()`.
Update all documentation. Closes #1371 and #1367.

NEWS.md

@@ -3,11 +3,15 @@ rearrangements of Notcurses.
 
 * 2.2.3 (not yet released)
   * Add `SIGILL` to the set of fatal signals we handle.
+  * Added `NCKEY_SIGNAL`. `NCKEY_RESIZE` is now an alias for `NCKEY_SIGNAL`.
+  * `SIGCONT` now synthesizes a `NCKEY_SIGNAL`.
   * Add the `nctree` widget for line-oriented hierarchical data. See
     the new `notcurses_tree(3)` man page for complete information.
   * Implemented `NCBLIT_PIXEL` for terminals reporting Sixel support.
-    Attempt Sixel detection iff `NCOPTION_VERIFY_SIXEL` is provided to
-    `notcurses_init()`. Sixel detection can delay or even block initialization.
+    Added `notcurses_check_pixel_support()` and its companion
+    `ncdirect_check_pixel_support()`, which must be called (and must return
+    success) before `NCBLIT_PIXEL` will be available. `NCBLIT_PIXEL` degrades
+    to `NCBLIT_3x2` until support is verified.
 
 * 2.2.2 (2021-02-18):
   * `notcurses_stats()` no longer qualifies its `notcurses*` argument with

USAGE.md

@@ -91,11 +91,12 @@ typedef enum {
 // doing something weird (setting a locale not based on LANG).
 #define NCOPTION_INHIBIT_SETLOCALE   0x0001
 
-// Checking for Sixel support requires writing an escape, and then reading an
-// inline reply from the terminal. Since this can interact poorly with actual
-// user input, it's not done unless Sixel will actually be used. Set this flag
-// to unconditionally test for Sixel support in notcurses_init().
-#define NCOPTION_VERIFY_SIXEL        0x0002
+// Checking for pixel support might require writing a control sequence, and
+// then reading a reply directly from the terminal. If the terminal doesn't
+// support this, the application will lock up. If you'll be using pixels, set
+// this flag to perform the check in notcurses_init(). You must otherwise call
+// notcurses_check_pixel() before NCBLIT_PIXEL will become available.
+#define NCOPTION_VERIFY_PIXEL        0x0002ull
 
 // We typically install a signal handler for SIGWINCH that generates a resize
 // event in the notcurses_getc() queue. Set to inhibit this handler.
@@ -306,6 +307,11 @@ bool notcurses_cansextants(const struct notcurses* nc);
 
 // Can we draw Braille? The Linux console cannot.
 bool notcurses_canbraille(const struct notcurses* nc);
+
+// If NCOPTION_VERIFY_PIXEL was not supplied to notcurses_init(), this
+// function must successfully return before NCBLIT_PIXEL is available. Returns
+// -1 on error, 0 if pixel mode is not supported, or 1 if it is supported.
+int notcurses_check_pixel(struct notcurses* nc);
 ```
 
 ## Direct mode

doc/man/man3/notcurses_direct.3.md

@@ -72,6 +72,8 @@ notcurses_direct - minimal notcurses instances for styling text
 
 **bool ncdirect_canutf8(const struct ncdirect* ***n***);**
 
+**int ncdirect_check_pixel_support(struct ncdirect* ***n***);**
+
 **int ncdirect_hline_interp(struct ncdirect* ***n***, const char* ***egc***, int ***len***, uint64_t ***h1***, uint64_t ***h2***);**
 
 **int ncdirect_vline_interp(struct ncdirect* ***n***, const char* ***egc***, int ***len***, uint64_t ***h1***, uint64_t ***h2***);**
@@ -144,6 +146,10 @@ information, consult **readline(3)**. If you want input echoed to the
 terminal while using **ncdirect_readline**, **NCDIRECT_OPTION_INHIBIT_CBREAK**
 must be supplied to **ncdirect_init**.
 
+**ncdirect_check_pixel_support** must be called (and successfully return)
+before **NCBLIT_PIXEL** can be used to render images; see
+**notcurses_visual(3)** for more details.
+
 # RETURN VALUES
 
 **ncdirect_init** returns **NULL** on failure. Otherwise, the return value
@@ -153,6 +159,9 @@ to **ncdirect_stop**.
 **ncdirect_putstr** and **ncdirect_printf_aligned** return the number of bytes
 written on success. On failure, they return some negative number.
 
+**ncdirect_check_pixel_support** returns -1 on error, 0 if there is no pixel
+support, and 1 if pixel support is successfully detected.
+
 All other functions return 0 on success, and non-zero on error.
 
 # SEE ALSO
@@ -161,5 +170,6 @@ All other functions return 0 on success, and non-zero on error.
 **readline(3)**
 **notcurses(3)**,
 **notcurses_plane(3)**,
+**notcurses_visual(3)**,
 **terminfo(5)**,
 **termios(3)**

doc/man/man3/notcurses_init.3.md

@@ -12,7 +12,6 @@ notcurses_init - initialize a notcurses instance
 
 ```c
 #define NCOPTION_INHIBIT_SETLOCALE   0x0001ull
-#define NCOPTION_VERIFY_SIXEL        0x0002ull
 #define NCOPTION_NO_WINCH_SIGHANDLER 0x0004ull
 #define NCOPTION_NO_QUIT_SIGHANDLERS 0x0008ull
 #define NCOPTION_SUPPRESS_BANNERS    0x0020ull
@@ -111,12 +110,6 @@ zero. The following flags are defined:
     the **LANG** environment variable. Your program should call **setlocale(3)**
     itself, usually as one of the first lines.
 
-* **NCOPTION_VERIFY_SIXEL**: Checking for Sixel support requires writing an
-    escape, and then reading an inline reply from the terminal. Since this can
-    interact poorly with actual user input, it's not done unless Sixel will
-    actually be used. Set this flag to unconditionally test for Sixel support
-    in **notcurses_init**.
-
 * **NCOPTION_NO_WINCH_SIGHANDLER**: A signal handler will usually be installed
     for **SIGWINCH**, resulting in **NCKEY_RESIZE** events being generated on
     input. With this flag, the handler will not be installed.

doc/man/man3/notcurses_visual.3.md

@@ -92,6 +92,8 @@ typedef int (*streamcb)(struct notcurses*, struct ncvisual*, void*);
 
 **int ncplane_qrcode(struct ncplane* ***n***, int* ***ymax***, int* ***xmax***, const void* ***data***, size_t ***len***)**
 
+**int notcurses_check_pixel_support(struct notcurses* ***nc***);**
+
 # DESCRIPTION
 
 An **ncvisual** is a virtual pixel framebuffer. They can be created from
@@ -203,6 +205,16 @@ Finally, rendering operates slightly differently when two planes have both been
 blitted, and one lies atop the other. See **notcurses_render(3)** for more
 information.
 
+# PIXEL BLITTING
+
+Some terminals support pixel-based output, either via Sixel or some bespoke
+mechanism. Checking for Sixel support requires interrogating the terminal and
+reading a response. This takes time, and will never complete if the terminal
+doesn't respond. Before **NCBLIT_PIXEL** can be used, it is thus necessary to
+check for support with **notcurses_check_pixel_support**. If this function has
+not successfully returned, attempts to use **NCBLIT_PIXEL** will fall back to
+**NCBLIT_3x2** (or fail, if **NCVISUAL_OPTION_NODEGRADE** is used).
+
 # RETURN VALUES
 
 **ncvisual_from_file** returns an **ncvisual** object on success, or **NULL**

include/ncpp/Direct.hh

@@ -217,6 +217,11 @@ namespace ncpp
 			return ncdirect_canutf8 (direct);
 		}
 
+		int check_pixel_support() noexcept
+		{
+			return ncdirect_check_pixel_support (direct);
+		}
+
 	private:
 		ncdirect *direct;
 	};

include/notcurses/direct.h

@@ -216,6 +216,11 @@ API bool ncdirect_canopen_images(const struct ncdirect* n);
 // Is our encoding UTF-8? Requires LANG being set to a UTF8 locale.
 API bool ncdirect_canutf8(const struct ncdirect* n);
 
+// This function must successfully return before NCBLIT_PIXEL is available.
+// Returns -1 on error, 0 for no support, or 1 if pixel output is supported.
+// Must not be called concurrently with either input or rasterization.
+API int ncdirect_check_pixel_support(struct ncdirect* n);
+
 // Draw horizontal/vertical lines using the specified channels, interpolating
 // between them as we go. The EGC may not use more than one column. For a
 // horizontal line, |len| cannot exceed the screen width minus the cursor's

include/notcurses/nckeys.h

@@ -9,7 +9,7 @@ extern "C" {
 
 // Special composed key definitions. These values are added to 0x100000.
 #define NCKEY_INVALID suppuabize(0)
-#define NCKEY_SIGNAL  suppuabize(1) // generated internally in response to SIGWINCH / SIGCONT
+#define NCKEY_SIGNAL  suppuabize(1) // we received either SIGWINCH or SIGCONT
 #define NCKEY_UP      suppuabize(2)
 #define NCKEY_RIGHT   suppuabize(3)
 #define NCKEY_DOWN    suppuabize(4)

include/notcurses/notcurses.h

@@ -826,11 +826,7 @@ typedef enum {
 // doing something weird (setting a locale not based on LANG).
 #define NCOPTION_INHIBIT_SETLOCALE   0x0001ull
 
-// Checking for Sixel support requires writing an escape, and then reading an
-// inline reply from the terminal. Since this can interact poorly with actual
-// user input, it's not done unless Sixel will actually be used. Set this flag
-// to unconditionally test for Sixel support in notcurses_init().
-#define NCOPTION_VERIFY_SIXEL        0x0002ull
+// NCOPTION_VERIFY_PIXEL was removed in 2.2.3. It ought be repurposed. FIXME.
 
 // We typically install a signal handler for SIGWINCH that generates a resize
 // event in the notcurses_getc() queue. Set to inhibit this handler.
@@ -1237,6 +1233,11 @@ API bool notcurses_canbraille(const struct notcurses* nc);
 // Can we blit to pixel graphics?
 API bool notcurses_canpixel(const struct notcurses* nc);
 
+// This function must successfully return before NCBLIT_PIXEL is available.
+// Returns -1 on error, 0 for no support, or 1 if pixel output is supported.
+// Must not be called concurrently with either input or rasterization.
+API int notcurses_check_pixel_support(struct notcurses* nc);
+
 typedef struct ncstats {
   // purely increasing stats
   uint64_t renders;          // successful ncpile_render() runs

src/lib/direct.cpp

@@ -637,6 +637,7 @@ ncdirect_stop_minimal(void* vnc){
     ret |= close(nc->ctermfd);
   }
   ret |= ncdirect_flush(nc);
+  free_terminfo_cache(&nc->tcache);
   return ret;
 }
 
@@ -1102,3 +1103,13 @@ int ncdirect_flush(const ncdirect* nc){
   }
   return 0;
 }
+
+int ncdirect_check_pixel_support(ncdirect* n){
+  if(query_term(&n->tcache, n->ctermfd)){
+    return -1;
+  }
+  if(n->tcache.pixelon){
+    return 1;
+  }
+  return 0;
+}

src/lib/internal.h

@@ -286,11 +286,12 @@ typedef struct tinfo {
   // background_opaque is in use. detect this, and avoid the default if so.
   // bg_collides_default is either 0x0000000 or 0x1RRGGBB.
   uint32_t bg_collides_default;
-  bool sextants;  // do we have (good, vetted) Unicode 13 sextant support?
-  bool braille;   // do we have Braille support? (linux console does not)
+  pthread_mutex_t pixel_query; // only query for pixel support once
   char* pixelon;  // enter pixel graphics mode
   char* pixeloff; // leave pixel graphics mode
-  bool sixel;     // do we have Sixel support?
+  bool pixel_query_done; // have we yet performed pixel query?
+  bool sextants;  // do we have (good, vetted) Unicode 13 sextant support?
+  bool braille;   // do we have Braille support? (linux console does not)
 } tinfo;
 
 typedef struct ncinputlayer {
@@ -386,6 +387,8 @@ int terminfostr(char** gseq, const char* name);
 // initialized. set |utf8| if we've verified UTF8 output encoding.
 int interrogate_terminfo(tinfo* ti, const char* termname, unsigned utf8);
 
+void free_terminfo_cache(tinfo* ti);
+
 // perform queries that require writing to the terminal, and reading a
 // response, rather than simply reading the terminfo database. can result
 // in a lengthy delay or even block if the terminal doesn't respond.

src/lib/notcurses.c

@@ -777,14 +777,13 @@ init_banner(const notcurses* nc){
     term_fg_palindex(nc, stdout, nc->tcache.colors <= 256 ? 50 % nc->tcache.colors : 0x20e080);
     printf("\n notcurses %s by nick black et al", notcurses_version());
     term_fg_palindex(nc, stdout, nc->tcache.colors <= 256 ? 12 % nc->tcache.colors : 0x2080e0);
-    printf("\n  %d rows %d cols (%sB) %zuB cells %d colors%s%s\n"
+    printf("\n  %d rows %d cols (%sB) %zuB cells %d colors%s\n"
            "  compiled with gcc-%s, %s-endian\n"
            "  terminfo from %s\n",
            nc->stdplane->leny, nc->stdplane->lenx,
            bprefix(nc->stats.fbbytes, 1, prefixbuf, 0), sizeof(nccell),
            nc->tcache.colors,
            nc->tcache.RGBflag ? "+RGB" : "",
-           nc->tcache.sixel ? "+Sixel" : "",
            __VERSION__,
 #ifdef __BYTE_ORDER__
 #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
@@ -908,6 +907,16 @@ recursive_lock_init(pthread_mutex_t *lock){
 #endif
 }
 
+int notcurses_check_pixel_support(notcurses* nc){
+  if(query_term(&nc->tcache, nc->ttyfd)){
+    return -1;
+  }
+  if(nc->tcache.pixelon){
+    return 1;
+  }
+  return 0;
+}
+
 // FIXME cut this up into a few distinct pieces, yearrrgh
 notcurses* notcurses_core_init(const notcurses_options* opts, FILE* outfp){
   notcurses_options defaultopts;
@@ -1033,12 +1042,6 @@ notcurses* notcurses_core_init(const notcurses_options* opts, FILE* outfp){
     goto err;
   }
   if(ret->ttyfd >= 0){
-    if(opts->flags & NCOPTION_VERIFY_SIXEL){
-      if(query_term(&ret->tcache, ret->ttyfd)){
-        free_plane(ret->stdplane);
-        goto err;
-      }
-    }
     if(ret->tcache.smkx && tty_emit(ret->tcache.smkx, ret->ttyfd)){
       free_plane(ret->stdplane);
       goto err;
@@ -1215,6 +1218,7 @@ int notcurses_stop(notcurses* nc){
     del_curterm(cur_term);
     ret |= pthread_mutex_destroy(&nc->statlock);
     ret |= pthread_mutex_destroy(&nc->pilelock);
+    free_terminfo_cache(&nc->tcache);
     free(nc);
   }
   return ret;
@@ -2073,7 +2077,7 @@ bool notcurses_canchangecolor(const notcurses* nc){
 }
 
 bool notcurses_canpixel(const notcurses* nc){
-  return nc->tcache.sixel;
+  return nc->tcache.pixelon;
 }
 
 palette256* palette256_new(notcurses* nc){

src/lib/terminfo.c

@@ -71,6 +71,10 @@ apply_term_heuristics(tinfo* ti, const char* termname){
   return 0;
 }
 
+void free_terminfo_cache(tinfo* ti){
+  pthread_mutex_destroy(&ti->pixel_query);
+}
+
 // termname is just the TERM environment variable. some details are not
 // exposed via terminfo, and we must make heuristic decisions based on
 // the detected terminal type, yuck :/.
@@ -184,6 +188,8 @@ int interrogate_terminfo(tinfo* ti, const char* termname, unsigned utf8){
     ti->fgop = "\x1b[39m";
     ti->bgop = "\x1b[49m";
   }
+  pthread_mutex_init(&ti->pixel_query, NULL);
+  ti->pixel_query_done = false;
   if(apply_term_heuristics(ti, termname)){
     return -1;
   }
@@ -229,8 +235,8 @@ query_sixel(tinfo* ti, int fd){
           state = DONE;
         }else if(in == '4'){
           if(!ti->pixelon){
-            ti->pixelon = strdup("\ePq");
-            ti->pixeloff = strdup("\e\\");
+            ti->pixelon = "\ePq";
+            ti->pixeloff = "\e\\";
           } // FIXME else warning?
         }
         break;
@@ -245,10 +251,18 @@ query_sixel(tinfo* ti, int fd){
   return 0;
 }
 
-// fd must be a real terminal, and must not be in nonblocking mode
+// fd must be a real terminal, and must not be in nonblocking mode. uses the
+// pthread_mutex_t of |ti| to only act once.
 int query_term(tinfo* ti, int fd){
-  if(query_sixel(ti, fd)){
+  if(fd < 0){
     return -1;
   }
-  return 0;
+  int ret = 0;
+  pthread_mutex_lock(&ti->pixel_query);
+  if(!ti->pixel_query_done){
+    ret = query_sixel(ti, fd);
+    ti->pixel_query_done = true;
+  }
+  pthread_mutex_unlock(&ti->pixel_query);
+  return ret;
 }

src/player/play.cpp

@@ -143,6 +143,9 @@ auto perframe(struct ncvisual* ncv, struct ncvisual_options* vopts,
     }else if(keyp >= '0' && keyp <= '8'){ // FIXME eliminate ctrl/alt
       marsh->blitter = static_cast<ncblitter_e>(keyp - '0');
       vopts->blitter = marsh->blitter;
+      if(vopts->blitter == NCBLIT_PIXEL){
+        notcurses_check_pixel_support(nc);
+      }
       continue;
     }else if(keyp == NCKey::Up){
       // FIXME
@@ -280,6 +283,9 @@ int direct_mode_player(int argc, char** argv, ncscale_e scalemode, ncblitter_e b
     return -1;
   }
   bool failed = false;
+  if(blitter == NCBLIT_PIXEL){
+    dm.check_pixel_support();
+  }
   {
     for(auto i = 0 ; i < argc ; ++i){
       try{
@@ -303,7 +309,6 @@ auto main(int argc, char** argv) -> int {
   float timescale, displaytime;
   ncscale_e scalemode;
   notcurses_options ncopts{};
-  ncopts.flags = NCOPTION_VERIFY_SIXEL;
   ncblitter_e blitter = NCBLIT_PIXEL;
   bool quiet = false;
   bool loop = false;
@@ -348,6 +353,9 @@ auto main(int argc, char** argv) -> int {
       vopts.n = *stdn;
       vopts.scaling = scalemode;
       vopts.blitter = blitter;
+      if(vopts.blitter == NCBLIT_PIXEL){
+        notcurses_check_pixel_support(nc);
+      }
       do{
         struct marshal marsh = {
           .subtitle_plane = nullptr,