string: add LineReader

diff --git a/src/test/util_string_tests.cpp b/src/test/util_string_tests.cpp
index 4a49e5d0..9f7513fb 100644
--- a/src/test/util_string_tests.cpp
+++ b/src/test/util_string_tests.cpp
@@ -4,6 +4,7 @@
 
 #include <util/strencodings.h>
 #include <util/string.h>
+#include <vector>
 
 #include <boost/test/unit_test.hpp>
 #include <test/util/setup_common.h>
@@ -41,6 +42,12 @@ void FailFmtWithError(const char* wrong_fmt, std::string_view error)
     BOOST_CHECK_EXCEPTION(CheckNumFormatSpecifiers<WrongNumArgs>(wrong_fmt), const char*, HasReason{error});
 }
 
+std::vector<std::byte> StringToBuffer(const std::string& str)
+{
+    auto span = std::as_bytes(std::span(str));
+    return {span.begin(), span.end()};
+}
+
 BOOST_AUTO_TEST_CASE(ConstevalFormatString_NumSpec)
 {
     PassFmt<0>("");
@@ -181,4 +188,114 @@ BOOST_AUTO_TEST_CASE(ascii_case_insensitive_hash_test)
     BOOST_CHECK_EQUAL(hsh("A\xfe"), hsh("a\xfe"));
 }
 
+BOOST_AUTO_TEST_CASE(line_reader_test)
+{
+    {
+        // Check three lines terminated by \n and \r\n, trimming whitespace
+        const std::vector<std::byte> input{StringToBuffer("once upon a time\n there was a dog \r\nwho liked food\n")};
+        LineReader reader(input, /*max_line_length=*/128);
+        std::optional<std::string> line1{reader.ReadLine()};
+        BOOST_CHECK_EQUAL(reader.Remaining(), 34);
+        std::optional<std::string> line2{reader.ReadLine()};
+        BOOST_CHECK_EQUAL(reader.Remaining(), 15);
+        std::optional<std::string> line3{reader.ReadLine()};
+        std::optional<std::string> line4{reader.ReadLine()};
+        BOOST_CHECK(line1);
+        BOOST_CHECK(line2);
+        BOOST_CHECK(line3);
+        BOOST_CHECK(!line4);
+        BOOST_CHECK_EQUAL(line1.value(), "once upon a time");
+        BOOST_CHECK_EQUAL(line2.value(), "there was a dog");
+        BOOST_CHECK_EQUAL(line3.value(), "who liked food");
+    }
+    {
+        // Do not exceed max_line_length + 1 while searching for \n
+        // Test with 22-character line + \n + 23-character line + \n
+        const std::vector<std::byte> input{StringToBuffer("once upon a time there\nwas a dog who liked tea\n")};
+
+        LineReader reader1(input, /*max_line_length=*/22);
+        // First line is exactly the length of max_line_length
+        BOOST_CHECK_EQUAL(reader1.ReadLine(), "once upon a time there");
+        // Second line is +1 character too long
+        BOOST_CHECK_EXCEPTION(reader1.ReadLine(), std::runtime_error, HasReason{"max_line_length exceeded by LineReader"});
+
+        // Increase max_line_length by 1
+        LineReader reader2(input, /*max_line_length=*/23);
+        // Both lines fit within limit
+        BOOST_CHECK_EQUAL(reader2.ReadLine(), "once upon a time there");
+        BOOST_CHECK_EQUAL(reader2.ReadLine(), "was a dog who liked tea");
+        // End of buffer reached
+        BOOST_CHECK(!reader2.ReadLine());
+    }
+    {
+        // Empty lines are empty
+        const std::vector<std::byte> input{StringToBuffer("\n")};
+        LineReader reader(input, /*max_line_length=*/1024);
+        BOOST_CHECK_EQUAL(reader.ReadLine(), "");
+        BOOST_CHECK(!reader.ReadLine());
+    }
+    {
+        // Empty buffers are null
+        const std::vector<std::byte> input{StringToBuffer("")};
+        LineReader reader(input, /*max_line_length=*/1024);
+        BOOST_CHECK(!reader.ReadLine());
+    }
+    {
+        // Even one character is too long, if it's not \n
+        const std::vector<std::byte> input{StringToBuffer("ab\n")};
+        LineReader reader(input, /*max_line_length=*/1);
+        // First line is +1 character too long
+        BOOST_CHECK_EXCEPTION(reader.ReadLine(), std::runtime_error, HasReason{"max_line_length exceeded by LineReader"});
+    }
+    {
+        const std::vector<std::byte> input{StringToBuffer("a\nb\n")};
+        LineReader reader(input, /*max_line_length=*/1);
+        BOOST_CHECK_EQUAL(reader.ReadLine(), "a");
+        BOOST_CHECK_EQUAL(reader.ReadLine(), "b");
+        BOOST_CHECK(!reader.ReadLine());
+    }
+    {
+        // If ReadLine fails, the iterator is reset and we can ReadLength instead
+        const std::vector<std::byte> input{StringToBuffer("a\nbaboon\n")};
+        LineReader reader(input, /*max_line_length=*/1);
+        BOOST_CHECK_EQUAL(reader.ReadLine(), "a");
+        // "baboon" is too long
+        BOOST_CHECK_EXCEPTION(reader.ReadLine(), std::runtime_error, HasReason{"max_line_length exceeded by LineReader"});
+        BOOST_CHECK_EQUAL(reader.ReadLength(1), "b");
+        BOOST_CHECK_EQUAL(reader.ReadLength(1), "a");
+        BOOST_CHECK_EQUAL(reader.ReadLength(2), "bo");
+        // "on" is too long
+        BOOST_CHECK_EXCEPTION(reader.ReadLine(), std::runtime_error, HasReason{"max_line_length exceeded by LineReader"});
+        BOOST_CHECK_EQUAL(reader.ReadLength(1), "o");
+        BOOST_CHECK_EQUAL(reader.ReadLine(), "n"); // now the remainder of the buffer fits in one line
+        BOOST_CHECK(!reader.ReadLine());
+    }
+    {
+        // The end of the buffer (EOB) does not count as end of line \n
+        const std::vector<std::byte> input{StringToBuffer("once upon a time there")};
+
+        LineReader reader(input, /*max_line_length=*/22);
+        // First line is exactly the length of max_line_length, but that doesn't matter because \n is missing
+        BOOST_CHECK(!reader.ReadLine());
+        // Data can still be read using ReadLength
+        BOOST_CHECK_EQUAL(reader.ReadLength(22), "once upon a time there");
+        // End of buffer reached
+        BOOST_CHECK_EQUAL(reader.Remaining(), 0);
+    }
+    {
+        // Read specific number of bytes regardless of max_line_length or \n unless buffer is too short
+        const std::vector<std::byte> input{StringToBuffer("once upon a time\n there was a dog \r\nwho liked food")};
+        LineReader reader(input, /*max_line_length=*/1);
+        BOOST_CHECK_EQUAL(reader.ReadLength(0), "");
+        BOOST_CHECK_EQUAL(reader.ReadLength(3), "onc");
+        BOOST_CHECK_EQUAL(reader.ReadLength(8), "e upon a");
+        BOOST_CHECK_EQUAL(reader.ReadLength(8), " time\n t");
+        BOOST_CHECK_EXCEPTION(reader.ReadLength(128), std::runtime_error, HasReason{"Not enough data in buffer"});
+        // After the error the iterator is reset so we can try again
+        BOOST_CHECK_EQUAL(reader.ReadLength(31), "here was a dog \r\nwho liked food");
+        // End of buffer reached
+        BOOST_CHECK_EQUAL(reader.Remaining(), 0);
+    }
+}
+
 BOOST_AUTO_TEST_SUITE_END()
diff --git a/src/util/string.cpp b/src/util/string.cpp
index 507d9d31..c3b4b474 100644
--- a/src/util/string.cpp
+++ b/src/util/string.cpp
@@ -13,4 +13,58 @@ void ReplaceAll(std::string& in_out, const std::string& search, const std::strin
     if (search.empty()) return;
     in_out = std::regex_replace(in_out, std::regex(search), substitute);
 }
+
+LineReader::LineReader(std::span<const std::byte> buffer, size_t max_line_length)
+    : start(buffer.begin()), end(buffer.end()), max_line_length(max_line_length), it(buffer.begin()) {}
+
+std::optional<std::string> LineReader::ReadLine()
+{
+    if (it == end) {
+        return std::nullopt;
+    }
+
+    auto line_start = it;
+    size_t count = 0;
+    while (it != end) {
+        // Read a character from the incoming buffer and increment the iterator
+        auto c = static_cast<char>(*it);
+        ++it;
+        ++count;
+        // If the character we just consumed was \n, the line is terminated.
+        // The \n itself does not count against max_line_length.
+        if (c == '\n') {
+            const std::string_view untrimmed_line(reinterpret_cast<const char*>(std::to_address(line_start)), count);
+            const std::string_view line = TrimStringView(untrimmed_line); // delete leading and trailing whitespace including \r and \n
+            return std::string(line);
+        }
+        // If the character we just consumed gives us a line length greater
+        // than max_line_length, and we are not at the end of the line (or buffer) yet,
+        // that means the line we are currently reading is too long, and we throw.
+        if (count > max_line_length) {
+            // Reset iterator
+            it = line_start;
+            throw std::runtime_error("max_line_length exceeded by LineReader");
+        }
+    }
+    // End of buffer reached without finding a \n or exceeding max_line_length.
+    // Reset the iterator so the rest of the buffer can be read granularly
+    // with ReadLength() and return null to indicate a line was not found.
+    it = line_start;
+    return std::nullopt;
+}
+
+// Ignores max_line_length but won't overflow
+std::string LineReader::ReadLength(size_t len)
+{
+    if (len == 0) return "";
+    if (Remaining() < len) throw std::runtime_error("Not enough data in buffer");
+    std::string out(reinterpret_cast<const char*>(std::to_address(it)), len);
+    it += len;
+    return out;
+}
+
+size_t LineReader::Remaining() const
+{
+    return std::distance(it, end);
+}
 } // namespace util
diff --git a/src/util/string.h b/src/util/string.h
index 330c2a2a..048e83ba 100644
--- a/src/util/string.h
+++ b/src/util/string.h
@@ -11,6 +11,7 @@
 #include <cstdint>
 #include <cstring>
 #include <locale>
+#include <optional>
 #include <sstream>
 #include <string>      // IWYU pragma: export
 #include <string_view> // IWYU pragma: export
@@ -260,6 +261,40 @@ template <typename T1, size_t PREFIX_LEN>
     return obj.size() >= PREFIX_LEN &&
            std::equal(std::begin(prefix), std::end(prefix), std::begin(obj));
 }
+
+struct LineReader {
+    const std::span<const std::byte>::iterator start;
+    const std::span<const std::byte>::iterator end;
+    const size_t max_line_length;
+    std::span<const std::byte>::iterator it;
+
+    explicit LineReader(std::span<const std::byte> buffer, size_t max_line_length);
+
+    /**
+     * Returns a string from current iterator position up to (but not including) next \n
+     * and advances iterator to the character following the \n on success.
+     * Will not return a line longer than max_line_length.
+     * @returns the next string from the buffer.
+     *          std::nullopt if end of buffer is reached without finding a \n.
+     * @throws a std::runtime_error if max_line_length + 1 bytes are read without finding \n.
+     */
+    std::optional<std::string> ReadLine();
+
+    /**
+     * Returns string from current iterator position of specified length
+     * if possible and advances iterator on success.
+     * May exceed max_line_length but will not read past end of buffer.
+     * @param[in]   len     The number of bytes to read from the buffer
+     * @returns a string of the expected length.
+     * @throws a std::runtime_error if there is not enough data in the buffer.
+     */
+    std::string ReadLength(size_t len);
+
+    /**
+     * Returns remaining size of bytes in buffer
+     */
+    size_t Remaining() const;
+};
 } // namespace util
 
 #endif // BITCOIN_UTIL_STRING_H