YaML: escape exclamation marks to avoid issues with compliant parsers

and added some support for reading such escaped characters.

radio/src/storage/conversions/yaml/yaml_datastructs_funcs.cpp

@@ -437,7 +437,7 @@ bool w_swtchSrc(const YamlNode* node, uint32_t val, yaml_writer_func wf, void* o
 {
     int32_t sval = yaml_to_signed(val, node->size);
     if (sval < 0) {
-        wf(opaque, "!", 1);
+        wf(opaque, "\\!", 2);
         sval = abs(sval);
     }
 

radio/src/storage/yaml/yaml_datastructs_funcs.cpp

@@ -763,7 +763,7 @@ static bool w_swtchSrc(const YamlNode* node, uint32_t val, yaml_writer_func wf,
 {
     int32_t sval = yaml_to_signed(val, node->size);
     if (sval < 0) {
-        wf(opaque, "!", 1);
+        wf(opaque, "\\!", 2);
         sval = abs(sval);
     }
 

radio/src/storage/yaml/yaml_parser.cpp

@@ -213,6 +213,10 @@ YamlParser::parse(const char* buffer, unsigned int size)
                 state = ps_ValQuo;
                 break;
             }
+            if (*c == '\\') {
+                state = ps_ValEsc;
+                break;
+            }
             CONCAT_STR(scratch_buf, scratch_len, *c);
             break;
 
@@ -284,6 +288,15 @@ YamlParser::parse(const char* buffer, unsigned int size)
                 state = ps_ValQuo;
                 break;
             }
+            if (*c == '\\') {
+                state = ps_ValEsc;
+                break;
+            }
+            CONCAT_STR(scratch_buf, scratch_len, *c);
+            break;
+
+        case ps_ValEsc:
+            state = ps_Val;
             CONCAT_STR(scratch_buf, scratch_len, *c);
             break;
                 

radio/src/storage/yaml/yaml_parser.h

@@ -46,6 +46,7 @@ class YamlParser
         ps_AttrSP,
         ps_Sep,
         ps_Val,
+        ps_ValEsc,
         ps_ValQuo,
         ps_ValEsc1,
         ps_ValEsc2,