scalar: implement a minimal JSON parser

No grown-up C project comes without their own JSON parser.

Just kidding!

We need to parse a JSON result when determining which cache server to
use. It would appear that searching for needles `"CacheServers":[`,
`"Url":"` and `"GlobalDefault":true` _happens_ to work right now, it is
fragile as it depends on no whitespace padding and on the order of the
fields remaining as-is.

Let's implement a super simple JSON parser (at the cost of being
slightly inefficient) for that purpose. To avoid allocating a ton of
memory, we implement a callback-based one. And to save on complexity,
let's not even bother validating the input properly (we will just go
ahead and instead rely on Azure Repos to produce correct JSON).

Note: An alternative would have been to use existing solutions such as
JSON-C, CentiJSON or JSMN. However, they are all a lot larger than the
current solution; The smallest, JSMN, which does not even provide parsed
string values (something we actually need) weighs in with 471 lines,
while we get away with 182 + 29 lines for the C and the header file,
respectively.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Makefile

@@ -2725,6 +2725,7 @@ GIT_OBJS += git.o
 .PHONY: git-objs
 git-objs: $(GIT_OBJS)
 
+SCALAR_OBJS := json-parser.o
 SCALAR_OBJS += scalar.o
 .PHONY: scalar-objs
 scalar-objs: $(SCALAR_OBJS)
@@ -2873,7 +2874,7 @@ $(REMOTE_CURL_PRIMARY): remote-curl.o http.o http-walker.o $(LAZYLOAD_LIBCURL_OB
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) $(filter %.o,$^) \
 		$(CURL_LIBCURL) $(EXPAT_LIBEXPAT) $(LIBS)
 
-scalar$X: scalar.o GIT-LDFLAGS $(GITLIBS)
+scalar$X: $(SCALAR_OBJS) GIT-LDFLAGS $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ $(ALL_LDFLAGS) \
 		$(filter %.o,$^) $(LIBS)
 

contrib/buildsystems/CMakeLists.txt

@@ -797,7 +797,7 @@ target_link_libraries(git-sh-i18n--envsubst common-main)
 add_executable(git-shell ${CMAKE_SOURCE_DIR}/shell.c)
 target_link_libraries(git-shell common-main)
 
-add_executable(scalar ${CMAKE_SOURCE_DIR}/scalar.c)
+add_executable(scalar ${CMAKE_SOURCE_DIR}/scalar.c ${CMAKE_SOURCE_DIR}/json-parser.c)
 target_link_libraries(scalar common-main)
 
 if(CURL_FOUND)

json-parser.c

@@ -0,0 +1,183 @@
+#include "git-compat-util.h"
+#include "hex.h"
+#include "json-parser.h"
+
+static int reset_iterator(struct json_iterator *it)
+{
+	it->p = it->begin = it->json;
+	strbuf_release(&it->key);
+	strbuf_release(&it->string_value);
+	it->type = JSON_NULL;
+	return -1;
+}
+
+static int parse_json_string(struct json_iterator *it, struct strbuf *out)
+{
+	const char *begin = it->p;
+
+	if (*(it->p)++ != '"')
+		return error("expected double quote: '%.*s'", 5, begin),
+			reset_iterator(it);
+
+	strbuf_reset(&it->string_value);
+#define APPEND(c) strbuf_addch(out, c)
+	while (*it->p != '"') {
+		switch (*it->p) {
+		case '\0':
+			return error("incomplete string: '%s'", begin),
+				reset_iterator(it);
+		case '\\':
+			it->p++;
+			if (*it->p == '\\' || *it->p == '"')
+				APPEND(*it->p);
+			else if (*it->p == 'b')
+				APPEND(8);
+			else if (*it->p == 't')
+				APPEND(9);
+			else if (*it->p == 'n')
+				APPEND(10);
+			else if (*it->p == 'f')
+				APPEND(12);
+			else if (*it->p == 'r')
+				APPEND(13);
+			else if (*it->p == 'u') {
+				unsigned char binary[2];
+				int i;
+
+				if (hex_to_bytes(binary, it->p + 1, 2) < 0)
+					return error("invalid: '%.*s'",
+						     6, it->p - 1),
+						reset_iterator(it);
+				it->p += 4;
+
+				i = (binary[0] << 8) | binary[1];
+				if (i < 0x80)
+					APPEND(i);
+				else if (i < 0x0800) {
+					APPEND(0xc0 | ((i >> 6) & 0x1f));
+					APPEND(0x80 | (i & 0x3f));
+				} else if (i < 0x10000) {
+					APPEND(0xe0 | ((i >> 12) & 0x0f));
+					APPEND(0x80 | ((i >> 6) & 0x3f));
+					APPEND(0x80 | (i & 0x3f));
+				} else {
+					APPEND(0xf0 | ((i >> 18) & 0x07));
+					APPEND(0x80 | ((i >> 12) & 0x3f));
+					APPEND(0x80 | ((i >> 6) & 0x3f));
+					APPEND(0x80 | (i & 0x3f));
+				}
+			}
+			break;
+		default:
+			APPEND(*it->p);
+		}
+		it->p++;
+	}
+
+	it->end = it->p++;
+	return 0;
+}
+
+static void skip_whitespace(struct json_iterator *it)
+{
+	while (isspace(*it->p))
+		it->p++;
+}
+
+int iterate_json(struct json_iterator *it)
+{
+	skip_whitespace(it);
+	it->begin = it->p;
+
+	switch (*it->p) {
+	case '\0':
+		return reset_iterator(it), 0;
+	case 'n':
+		if (!starts_with(it->p, "null"))
+			return error("unexpected value: %.*s", 4, it->p),
+				reset_iterator(it);
+		it->type = JSON_NULL;
+		it->end = it->p = it->begin + 4;
+		break;
+	case 't':
+		if (!starts_with(it->p, "true"))
+			return error("unexpected value: %.*s", 4, it->p),
+				reset_iterator(it);
+		it->type = JSON_TRUE;
+		it->end = it->p = it->begin + 4;
+		break;
+	case 'f':
+		if (!starts_with(it->p, "false"))
+			return error("unexpected value: %.*s", 5, it->p),
+				reset_iterator(it);
+		it->type = JSON_FALSE;
+		it->end = it->p = it->begin + 5;
+		break;
+	case '-': case '.':
+	case '0': case '1': case '2': case '3': case '4':
+	case '5': case '6': case '7': case '8': case '9':
+		it->type = JSON_NUMBER;
+		it->end = it->p = it->begin + strspn(it->p, "-.0123456789");
+		break;
+	case '"':
+		it->type = JSON_STRING;
+		if (parse_json_string(it, &it->string_value) < 0)
+			return -1;
+		break;
+	case '[': {
+		const char *save = it->begin;
+		size_t key_offset = it->key.len;
+		int i = 0, res;
+
+		for (it->p++, skip_whitespace(it); *it->p != ']'; i++) {
+			strbuf_addf(&it->key, "[%d]", i);
+
+			if ((res = iterate_json(it)))
+				return reset_iterator(it), res;
+			strbuf_setlen(&it->key, key_offset);
+
+			skip_whitespace(it);
+			if (*it->p == ',')
+				it->p++;
+		}
+
+		it->type = JSON_ARRAY;
+		it->begin = save;
+		it->end = it->p;
+		it->p++;
+		break;
+	}
+	case '{': {
+		const char *save = it->begin;
+		size_t key_offset = it->key.len;
+		int res;
+
+		strbuf_addch(&it->key, '.');
+		for (it->p++, skip_whitespace(it); *it->p != '}'; ) {
+			strbuf_setlen(&it->key, key_offset + 1);
+			if (parse_json_string(it, &it->key) < 0)
+				return -1;
+			skip_whitespace(it);
+			if (*(it->p)++ != ':')
+				return error("expected colon: %.*s", 5, it->p),
+					reset_iterator(it);
+
+			if ((res = iterate_json(it)))
+				return res;
+
+			skip_whitespace(it);
+			if (*it->p == ',')
+				it->p++;
+		}
+		strbuf_setlen(&it->key, key_offset);
+
+		it->type = JSON_OBJECT;
+		it->begin = save;
+		it->end = it->p;
+		it->p++;
+		break;
+	}
+	}
+
+	return it->fn(it);
+}

json-parser.h

@@ -0,0 +1,29 @@
+#ifndef JSON_PARSER_H
+#define JSON_PARSER_H
+
+#include "strbuf.h"
+
+struct json_iterator {
+	const char *json, *p, *begin, *end;
+	struct strbuf key, string_value;
+	enum {
+		JSON_NULL = 0,
+		JSON_FALSE,
+		JSON_TRUE,
+		JSON_NUMBER,
+		JSON_STRING,
+		JSON_ARRAY,
+		JSON_OBJECT
+	} type;
+	int (*fn)(struct json_iterator *it);
+	void *fn_data;
+};
+#define JSON_ITERATOR_INIT(json_, fn_, fn_data_) { \
+	.json = json_, .p = json_, \
+	.key = STRBUF_INIT, .string_value = STRBUF_INIT, \
+	.fn = fn_, .fn_data = fn_data_ \
+}
+
+int iterate_json(struct json_iterator *it);
+
+#endif