song/Filter: operator "==" never searches substrings in filter expressions

The protocol documentation says that the difference between `find` and
`search` is that `search` is case insensitive, but that's only half
the truth: `search` also searches for sub strings instead of matching
the whole string.  This part is undocumented and unfortunate, but at
this point, we can't change it.

However leaking this surprising behavior to the new filter expressions
was a bad idea; the "==" operator should never match substrings.  For
people who need that, we should add a new operator.

NEWS

@@ -1,6 +1,7 @@
 ver 0.21.1 (not yet released)
 * protocol
   - allow escaping quotes in filter expressions
+  - operator "==" never searches substrings in filter expressions
 * decoder
   - ffmpeg: fix build failure with non-standard FFmpeg installation path
   - flac: fix linker failure when building without FLAC support

src/song/Filter.cxx

@@ -91,8 +91,11 @@ locate_parse_type(const char *str) noexcept
 
 SongFilter::SongFilter(TagType tag, const char *value, bool fold_case)
 {
+	/* for compatibility with MPD 0.20 and older, "fold_case" also
+	   switches on "substring" */
 	and_filter.AddItem(std::make_unique<TagSongFilter>(tag, value,
-							   fold_case, false));
+							   fold_case, fold_case,
+							   false));
 }
 
 SongFilter::~SongFilter()
@@ -296,11 +299,13 @@ SongFilter::ParseExpression(const char *&s, bool fold_case)
 		if (type == LOCATE_TAG_FILE_TYPE)
 			return std::make_unique<UriSongFilter>(std::move(value),
 							       fold_case,
+							       false,
 							       negated);
 
 		return std::make_unique<TagSongFilter>(TagType(type),
 						       std::move(value),
-						       fold_case, negated);
+						       fold_case, false,
+						       negated);
 	}
 }
 
@@ -325,7 +330,10 @@ SongFilter::Parse(const char *tag_string, const char *value, bool fold_case)
 		break;
 
 	case LOCATE_TAG_FILE_TYPE:
+		/* for compatibility with MPD 0.20 and older,
+		   "fold_case" also switches on "substring" */
 		and_filter.AddItem(std::make_unique<UriSongFilter>(value,
+								   fold_case,
 								   fold_case,
 								   false));
 		break;
@@ -334,9 +342,12 @@ SongFilter::Parse(const char *tag_string, const char *value, bool fold_case)
 		if (tag == LOCATE_TAG_ANY_TYPE)
 			tag = TAG_NUM_OF_ITEM_TYPES;
 
+		/* for compatibility with MPD 0.20 and older,
+		   "fold_case" also switches on "substring" */
 		and_filter.AddItem(std::make_unique<TagSongFilter>(TagType(tag),
 								   value,
 								   fold_case,
+								   fold_case,
 								   false));
 		break;
 	}

src/song/StringFilter.hxx

@@ -40,12 +40,12 @@ class StringFilter {
 
 public:
 	template<typename V>
-	StringFilter(V &&_value, bool _fold_case)
+	StringFilter(V &&_value, bool _fold_case, bool _substring)
 		:value(std::forward<V>(_value)),
 		 fold_case(_fold_case
 			   ? IcuCompare(value.c_str())
 			   : IcuCompare()),
-		 substring(_fold_case) {}
+		 substring(_substring) {}
 
 	bool empty() const noexcept {
 		return value.empty();

src/song/TagSongFilter.hxx

@@ -42,9 +42,10 @@ class TagSongFilter final : public ISongFilter {
 
 public:
 	template<typename V>
-	TagSongFilter(TagType _type, V &&_value, bool fold_case, bool _negated)
+	TagSongFilter(TagType _type, V &&_value, bool fold_case, bool substring,
+		      bool _negated)
 		:type(_type), negated(_negated),
-		 filter(std::forward<V>(_value), fold_case) {}
+		 filter(std::forward<V>(_value), fold_case, substring) {}
 
 	TagType GetTagType() const {
 		return type;

src/song/UriSongFilter.hxx

@@ -32,8 +32,9 @@ class UriSongFilter final : public ISongFilter {
 
 public:
 	template<typename V>
-	UriSongFilter(V &&_value, bool fold_case, bool _negated)
-		:filter(std::forward<V>(_value), fold_case),
+	UriSongFilter(V &&_value, bool fold_case, bool substring,
+		      bool _negated)
+		:filter(std::forward<V>(_value), fold_case, substring),
 		 negated(_negated) {}
 
 	const auto &GetValue() const noexcept {