Method mremap

core/src/main/java/io/questdb/std/MmapCache.java:129–243

Resizes existing memory mapping, reusing or creating new mapping as needed.

(int fd, long mmapCacheKey, long address, long previousSize, long newSize, long offset, int flags, int memoryTag)

     * Resizes existing memory mapping, reusing or creating new mapping as needed.
     */
    public long mremap(int fd, long mmapCacheKey, long address, long previousSize, long newSize, long offset, int flags, int memoryTag) {
        if (newSize < 1) {
            throw CairoException.critical(0)
                    .put("could not remap file, invalid newSize [previousSize=").put(previousSize)
                    .put(", newSize=").put(newSize)
                    .put(", offset=").put(offset)
                    .put(", fd=").put(fd)
                    .put(", memoryTag=").put(memoryTag)
                    .put(']');
        }
        if (offset != 0 || mmapCacheKey == 0 || !Files.FS_CACHE_ENABLED || flags == Files.MAP_RW) {
            return mremap0(fd, address, previousSize, newSize, offset, flags, memoryTag, memoryTag);
        }
        if (previousSize == 0) {
            // If previous size is 0, we cannot remap, just mmap a new region
            return cacheMmap(fd, mmapCacheKey, newSize, offset, flags, memoryTag);
        }

        long unmapPtr = 0, unmapLen = 0;
        int unmapTag = 0;
        long newAddress = 0;

        synchronized (this) {

            int addrMapIndex = mmapAddrCache.keyIndex(address);
            assert addrMapIndex < 0 : "old address is not found in mmap cache";

            MmapCacheRecord record = mmapAddrCache.valueAt(addrMapIndex);

            int fdIndex = Integer.MAX_VALUE;
            if (newSize >= previousSize) {
                if (record.length >= newSize) {
                    // Address is already long enough, just return it
                    return address;
                }

                // Check if someone else remapped this to a larger size
                fdIndex = mmapFileCache.keyIndex(mmapCacheKey);
                if (fdIndex < 0) {
                    MmapCacheRecord updatedCacheRecord = mmapFileCache.valueAt(fdIndex);
                    if (updatedCacheRecord.length >= newSize) {
                        // Cache for the FD is updated by someone else
                        // The fd cache record is already long enough, just return the address
                        updatedCacheRecord.count++;
                        newAddress = updatedCacheRecord.address;
                        // We should not store zero addresses in the cache. We do not cache if someone maps 0 length,
                        // and we do not allow to remap to 0 length.
                        assert newAddress != 0;
                        mmapReuseCount++;

                        record.count--;
                        if (record.count == 0) {
                            // The old cache record is not used anymore
                            mmapAddrCache.removeAt(addrMapIndex);
                            unmapPtr = record.address;
                            unmapLen = record.length;
                            unmapTag = record.memoryTag;
                            record.address = 0;