Better PNG load error reporting

kitty/graphics.c

@@ -305,7 +305,7 @@ err:
 }
 
 static void
-png_error_handler(const char *code, const char *msg) {
+png_error_handler(png_read_data *d UNUSED, const char *code, const char *msg) {
     set_command_failed_response(code, "%s", msg);
 }
 
@@ -333,6 +333,17 @@ add_trim_predicate(Image *img) {
     return !img->root_frame_data_loaded || (!img->client_id && !img->refs);
 }
 
+static void
+print_png_read_error(png_read_data *d, const char *code, const char* msg) {
+    if (d->error.used >= d->error.capacity) {
+        size_t cap = MAX(2 * d->error.capacity, 1024 + d->error.used);
+        d->error.buf = realloc(d->error.buf, cap);
+        if (!d->error.buf) return;
+        d->error.capacity = cap;
+    }
+    d->error.used += snprintf(d->error.buf + d->error.used, d->error.capacity - d->error.used, "%s: %s ", code, msg);
+}
+
 bool
 png_from_file_pointer(FILE *fp, const char *path_for_error_messages, uint8_t** data, unsigned int* width, unsigned int* height, size_t* sz) {
     size_t capacity = 16*1024, pos = 0;
@@ -356,16 +367,16 @@ png_from_file_pointer(FILE *fp, const char *path_for_error_messages, uint8_t** d
             return false;
         }
     }
-    png_read_data d = {0};
+    png_read_data d = {.err_handler=print_png_read_error};
     inflate_png_inner(&d, buf, pos);
     free(buf);
     if (!d.ok) {
-        free(d.decompressed); free(d.row_pointers);
-        log_error("Failed to decode PNG image at: %s", path_for_error_messages);
+        log_error("Failed to decode PNG image at: %s with error: %s", path_for_error_messages, d.error.used > 0 ? d.error.buf : "");
+        free(d.decompressed); free(d.row_pointers); free(d.error.buf);
         return false;
     }
     *data = d.decompressed;
-    free(d.row_pointers);
+    free(d.row_pointers); free(d.error.buf);
     *sz = d.sz;
     *height = d.height; *width = d.width;
     return true;

kitty/png-reader.c

@@ -5,6 +5,7 @@
  * Distributed under terms of the GPL3 license.
  */
 
+#include "data-types.h"
 #include "png-reader.h"
 #include "cleanup.h"
 #include "state.h"
@@ -26,7 +27,7 @@ read_png_from_buffer(png_structp png, png_bytep out, png_size_t length) {
 
 struct custom_error_handler {
     jmp_buf jb;
-    png_error_handler_func err_handler;
+    png_read_data *d;
 };
 
 static void
@@ -34,7 +35,7 @@ read_png_error_handler(png_structp png_ptr, png_const_charp msg) {
     struct custom_error_handler *eh;
     eh = png_get_error_ptr(png_ptr);
     if (eh == NULL) fatal("read_png_error_handler: could not retrieve error handler");
-    if(eh->err_handler) eh->err_handler("EBADPNG", msg);
+    if(eh->d->err_handler) eh->d->err_handler(eh->d, "EBADPNG", msg);
     longjmp(eh->jb, 1);
 }
 
@@ -43,14 +44,14 @@ read_png_warn_handler(png_structp UNUSED png_ptr, png_const_charp msg) {
     if (global_state.debug_rendering) log_error("libpng WARNING: %s", msg);
 }
 
-#define ABRT(code, msg) { if(d->err_handler) d->err_handler(#code, msg); goto err; }
+#define ABRT(code, msg) { if(d->err_handler) d->err_handler(d, #code, msg); goto err; }
 
 void
 inflate_png_inner(png_read_data *d, const uint8_t *buf, size_t bufsz) {
     struct fake_file f = {.buf = buf, .sz = bufsz};
     png_structp png = NULL;
     png_infop info = NULL;
-    struct custom_error_handler eh = {.err_handler = d->err_handler};
+    struct custom_error_handler eh = {.d = d};
     png = png_create_read_struct(PNG_LIBPNG_VER_STRING, &eh, read_png_error_handler, read_png_warn_handler);
     if (!png) ABRT(ENOMEM, "Failed to create PNG read structure");
     info = png_create_info_struct(png);
@@ -131,8 +132,8 @@ err:
 }
 
 static void
-png_error_handler(const char *code, const char *msg) {
-    PyErr_Format(PyExc_ValueError, "[%s] %s", code, msg);
+png_error_handler(png_read_data *d UNUSED, const char *code, const char *msg) {
+    if (!PyErr_Occurred()) PyErr_Format(PyExc_ValueError, "[%s] %s", code, msg);
 }
 
 static PyObject*

kitty/png-reader.h

@@ -6,16 +6,25 @@
 
 #pragma once
 
-#include "data-types.h"
+#include <stdint.h>
+#include <stdbool.h>
 #include <png.h>
-typedef void(*png_error_handler_func)(const char*, const char*);
-typedef struct {
+
+typedef struct png_read_data png_read_data;
+
+typedef void(*png_error_handler_func)(png_read_data *d, const char*, const char*);
+
+typedef struct png_read_data {
     uint8_t *decompressed;
     bool ok;
     png_bytep *row_pointers;
     int width, height;
     size_t sz;
     png_error_handler_func err_handler;
+    struct {
+        char *buf;
+        size_t used, capacity;
+    } error;
 } png_read_data;
 
 void inflate_png_inner(png_read_data *d, const uint8_t *buf, size_t bufsz);