Method read

gpcontrib/gpcloud/src/s3key_reader.cpp:55–98

ret < len means EMPTY that's why it checks if leftLen is larger than *or equal to* len below[1], provides a chance ret is 0, which is smaller than len. Otherwise, other functions won't know when to read next buffer.

// that's why it checks if leftLen is larger than *or equal to* len below[1], provides a chance ret
// is 0, which is smaller than len. Otherwise, other functions won't know when to read next buffer.
uint64_t ChunkBuffer::read(char* buf, uint64_t len) {
    // GPDB abort signal stops s3_import(), this check is not needed if s3_import() every time calls
    // ChunkBuffer->Read() only once, otherwise(as we did in downstreamReader->read() for
    // decompression feature before), first call sets buffer to ReadyToFill, second call hangs.
    S3_CHECK_OR_DIE(!S3QueryIsAbortInProgress(), S3QueryAbort, "");

    UniqueLock statusLock(&this->statusMutex);
    while (this->status != ReadyToRead) {
        pthread_cond_wait(&this->statusCondVar, &this->statusMutex);
    }

    // Error is shared between all chunks.
    if (this->isError()) {
        return 0;
    }

    uint64_t leftLen = this->chunkDataSize - this->curChunkOffset;
    uint64_t lenToRead = std::min(len, leftLen);

    if (lenToRead != 0) {
        memcpy(buf, this->chunkData.data() + this->curChunkOffset, lenToRead);
    }

    if (len <= leftLen) {                   // [1]
        this->curChunkOffset += lenToRead;  // not empty
    } else {                                // empty, reset everything
        this->curChunkOffset = 0;

        if (!this->isEOF()) {
            // Release chunkData memory to reduce consumption.
            this->chunkData.release();

            this->status = ReadyToFill;

            Range range = this->offsetMgr.getNextOffset();
            this->curFileOffset = range.offset;
            this->chunkDataSize = range.length;

            pthread_cond_signal(&this->statusCondVar);
        }
    }

    return lenToRead;
}

// returning uint64_t(-1) means error
uint64_t ChunkBuffer::fill() {