lib: preferably feed w3m some UTF-8 input

w3m cannot cope with UTF-16 or UTF-32, so in at least those cases,
input needs to be crossconverted to UTF-8 by us before handoff.

References: DESK-3063, GXL-589

Author: jengelh

lib/rfbl.cpp

@@ -495,6 +495,51 @@ pid_t popenfd(const char *const *argv, int *fdinp, int *fdoutp,
 	return pid;
 }
 
+/**
+ * @fp:       file to emit to
+ * @src:      input data
+ * @cset:     character set of data in @src
+ *
+ * Convert @src to UTF-8 if needed (depends on @cset), and writeout to @fp.
+ * Updates @cset in case w3m should not be given the -I argument.
+ * Returns 0 for success; other values indicate an error condition.
+ */
+static int utf8_writeout(FILE *fp, const void *vsrc, size_t src_size, const char *&cset)
+{
+	auto src = const_cast<char *>(static_cast<const char *>(vsrc));
+	if (cset == nullptr || strcasecmp(cset, "utf8") == 0 ||
+	    strcasecmp(cset, "utf-8") == 0)
+		return fwrite(src, src_size, 1, fp) == 1 ? 0 : -1;
+	auto cd = iconv_open("utf-8", cset);
+	if (cd == iconv_t(-1)) {
+		/* Dunno how to translate, just feed it as-is to w3m */
+		cset = nullptr;
+		return 0;
+	}
+	auto cleanup = HX::make_scope_exit([&]() { iconv_close(cd); });
+	char buffer[4096];
+
+	/* Loop copied from iconvtext() */
+	while (src_size > 0) {
+		auto dst = buffer;
+		size_t dst_size = sizeof(buffer);
+		auto ret = iconv(cd, &src, &src_size, &dst, &dst_size);
+		if (ret != static_cast<size_t>(-1) || dst_size != sizeof(buffer)) {
+			if (fwrite(buffer, sizeof(buffer) - dst_size, 1, fp) != 1)
+				return -1;
+			continue;
+		}
+		if (src_size > 0) {
+			--src_size;
+			++src;
+		}
+		if (fwrite(buffer, sizeof(buffer) - dst_size, 1, fp) != 1)
+			return -1;
+	}
+	errno = 0;
+	return 0;
+}
+
 /**
  * Run an external HTML-to-text converter.
  *
@@ -520,7 +565,7 @@ int feed_w3m(const void *inbuf, size_t len, const char *cset,
 	if (fp == nullptr)
 		return -1;
 	auto cl1 = HX::make_scope_exit([&]() { unlink(filename.c_str()); });
-	if (fwrite(inbuf, len, 1, fp.get()) != 1)
+	if (utf8_writeout(fp.get(), inbuf, len, cset) != 0)
 		return -1;
 	fp.reset();
 	int fout = -1;