config: improve validation of color values, default alpha to 0xff

Reject color values that aren't in either RGB, or ARGB format. That
is, color values that aren't hexadecimal numbers with either 6 or 8
digits.

Also, if a color value is allowed to have an alpha component, and the
user left it out, default to 0xff (opaque) rather than 0x00 (fully
transparent).

Closes #1526

CHANGELOG.md

@@ -52,9 +52,21 @@
 ## Unreleased
 ### Added
 ### Changed
+
+* config: ARGB color values now default to opaque, rather than
+  transparent, when the alpha component has been left out
+  ([#1526][1526]).
+
+[1526]: https://codeberg.org/dnkl/foot/issues/1526
+
+
 ### Deprecated
 ### Removed
 ### Fixed
+
+* config: improved validation of color values.
+
+
 ### Security
 ### Contributors
 

config.c

@@ -618,18 +618,30 @@ value_to_enum(struct context *ctx, const char **value_map, int *res)
 }
 
 static bool NOINLINE
-value_to_color(struct context *ctx, uint32_t *color, bool allow_alpha)
+value_to_color(struct context *ctx, uint32_t *result, bool allow_alpha)
 {
-    if (!str_to_uint32(ctx->value, 16, color)) {
-        LOG_CONTEXTUAL_ERR("not a valid color value");
+    uint32_t color;
+    const size_t len = strlen(ctx->value);
+    const size_t component_count = len / 2;
+
+    if (!(len == 6 || (allow_alpha && len == 8)) ||
+        !str_to_uint32(ctx->value, 16, &color))
+    {
+        if (allow_alpha) {
+            LOG_CONTEXTUAL_ERR("color must be in either RGB or ARGB format");
+        } else {
+            LOG_CONTEXTUAL_ERR("color must be in RGB format");
+        }
+
         return false;
     }
 
-    if (!allow_alpha && (*color & 0xff000000) != 0) {
-        LOG_CONTEXTUAL_ERR("color value must not have an alpha component");
-        return false;
+    if (allow_alpha && component_count == 3) {
+        /* If user left out the alpha component, assume non-transparency */
+        color |= 0xff000000;
     }
 
+    *result = color;
     return true;
 }
 

tests/test-config.c

@@ -426,6 +426,10 @@ test_color(struct context *ctx, bool (*parse_fun)(struct context *ctx),
         {"ffffff", 0xffffff},
         {"ffffffff", 0xffffffff, !alpha_allowed},
         {"aabbccdd", 0xaabbccdd, !alpha_allowed},
+        {"00", 0, true},
+        {"0000", 0, true},
+        {"00000", 0, true},
+        {"000000000", 0, true},
         {"unittest-invalid-color", 0, true},
     };