Input Streams

lib/istream.h describes Dovecot’s input streams. Input streams can be stacked on top of each others as many times as wanted.

Input streams actually reading data:

  • file: Read data from fd using pread() for files and read() for non-files.

  • unix: Read data from UNIX socket. Similar to file, but supports receiving file descriptors.

  • data: Read data from memory.

Input stream filters:

  • concat: Concatenate multiple input streams together

  • chain: Chain multiple input streams together. Similar to istream-concat, but more istreams can be added after initialization and EOF needs to be explicitly added.

  • seekable: Make a number of (possibly non-seekable) input streams into a single seekable input stream. If all of the input streams are already seekable, a concat stream is created instead.

    • Usually the only non-seekable input streams are non-file fds, such as pipes or sockets.

  • crlf: Change all newlines to either LFs or CRLFs, by adding or removing CRs as necessary.

  • limit: Limit input stream’s length, so after reading a given number of bytes it returns EOF.

  • sized: Require istream’s length to be exactly the given size, or the last read returns error.

  • timeout: Fail the read when given timeout is reached.

  • try: Read from the first input stream that doesn’t fail with EINVAL.

  • tee: Fork an input stream to multiple streams that can be read independently.

  • multiplex: Multiplex-iostreams support multiple iostream channels inside a single parent istream.

  • callback: Build an input stream by calling callback functions that return the data.

  • base64-encoder, base64-decoder: Encode/decode base64.

  • failure-at: Insert a failure at the specified offset. This can be useful for testing.

  • hash: Calculate hash of the istream while it’s being read.

  • lib-compression/*: Read zlib/bzlib/lz4/zstd compressed data.

There are also various other less generic istreams. Especially lib-mail has many mail-related istreams.

Reading

i_stream_read() tries to read more data into the stream’s buffer. It returns:

  • -2: Nothing was read, because the buffer is full.

  • -1: Either input reached EOF, or read failed and stream_errno was set.

  • 0: Input stream is non-blocking, and no more input is available now.

  • >0: Number of new bytes read.

Reading from a stream doesn’t actually go forward in the stream, that needs to be done manually with i_stream_skip(). This makes it easy to read full data records into the stream directly instead of creating separate buffers. For example when reading line-based input you can keep reading input into the stream until you find LF and then just access the string directly from the input buffer. There are actually helper functions for this: i_stream_next_line() attempts to return the next line if available, i_stream_read_next_line() does the same but does a read to try to get the data.

Because more and more data can be read into the buffer, the buffer size is typically limited, and once this limit is reached read returns -2. The buffer size is given as parameter to the i_stream_create_*(), but filters often use their parent stream’s buffer size. The buffer size can be also changed with i_stream_set_max_buffer_size(). Figuring out what the buffer size should be depends on the situation. It should be large enough to contain all valid input, but small enough that users can’t cause a DoS by sending a too large record and having Dovecot eat up all the memory. If there’s no specific buffer size requirement, IO_BLOCK_SIZE is a good value to use.

Once read returns -1, the stream has reached EOF. stream->eof=TRUE is also set. In this situation it’s important to remember that there may still be data available in the buffer. If i_stream_have_bytes_left() returns FALSE, there really isn’t anything left to read. Also at EOF it’s important to check stream->stream_errno to see if the read failed.

Whenever i_stream_read() returns >0, all the existing pointers are potentially invalidated. v2.3+: When i_stream_read() returns <= 0, the data previously returned by i_stream_get_data() are still valid, preserved in “snapshots”. (<v2.3 may or may not have invalidated them.)

Example:

/* Read line-based data from file_fd. The buffer size has no limits. */
struct istream *input = i_stream_create_fd(file_fd, SIZE_MAX, FALSE);
const char *line;

/* Return the last line also even if it doesn't end with LF.
   This is generally a good idea when reading files (but not a good idea
   when reading commands from e.g. socket). */
i_stream_set_return_partial_line(input, TRUE);
while ((line = i_stream_read_next_line(input)) != NULL) {
  /* handle line */
}
i_stream_destroy(&input);

Internals

lib/istream-private.h describes the internal API that input streams need to implement. The methods that need to be implemented are:

  • read() is the most important function. It can also be tricky to get it completely bug-free. See the existing unit tests for other istreams and try to test the edge cases as well (such as ability to read one byte at a time and also with max buffer size of 1). When it needs to read from parent streams, try to use i_stream_read_memarea(parent) if possible so a new snapshot isn’t unnecessarily created (see the snapshot discussion below).

  • seek(v_offset, mark) seeks to given offset. The mark parameter is necessary only when it’s difficult to seek backwards in the stream, such as when reading compressed input.

  • sync() removes everything from internal buffers, so that if the underlying file has changed the changes get noticed immediately after sync.

  • get_size(exact) returns the size of the input stream, if it’s known. If exact=TRUE, the returned size must be the same how many bytes can be read from the input. If exact=FALSE, the size is mainly used to compare against another stat to see if the underlying input had changed. For example with compressed input the size could be the compressed size.

  • stat(exact) stats the file, filling as much of the fields as makes sense. st_size field is filled the same way as with get_size(), or set to -1 if it’s unknown.

  • switch_ioloop_to If there are any I/O loop items associated with the stream, move all of them to the provided/current ioloop.

  • snapshot(prev_snapshot) creates a snapshot of the data that is currently available via i_stream_get_data(), merges it with prev_snapshot (if any) and returns the merged snapshot (see below for more details).

There are some fields available. Below is a list of the most important ones. For a complete overview please see istream-private.h.

  • fd file descriptor being read by the stream.

  • buffer contains pointer to the data.

  • parent parent istream - for filter streams.

  • First skip bytes of the buffer are already skipped over (with i_stream_skip() or seeking).

  • Data up to pos bytes (beginning after skip) in the buffer are available with i_stream_get_data(). If pos=skip, it means there is no available data in the buffer.

If your input stream needs a write buffer, you can use some of the common helper functions and variables:

  • w_buffer contain the pointer where you can write data. It should be kept in sync with buffer.

  • buffer_size specifies the buffer’s size, and max_buffer_size the max. size the buffer can be grown to.

  • i_stream_try_alloc(wanted_size, size_r) can be used when you want to store wanted_bytes into w_buffer. If the buffer isn’t large enough for it, it’s grown if possible. The buffer isn’t grown above the stream’s max buffer size. The returned size_r specifies how many bytes are actually available for writing at stream->w_buffer + stream->pos.

  • i_stream_alloc(size) is like i_stream_try_alloc(), except it always succeeds allocating size bytes, even if it has to grow the buffer larger then the stream’s max buffer size.

  • Lower-level memory allocation functions:

    • i_stream_grow_buffer(bytes) grows the w_buffer by the given number of bytes, if possible. It won’t reach the stream’s current max buffer size. The caller must verify from buffer_size how large the buffer became as a result of this call.

    • i_stream_compress() attempts to compress the current w_buffer by removing already-skipped data with memmove(). If skip is 0, it does nothing. Note that this function must not be called if memarea has refcount>1. Otherwise that could be modifying a snapshotted memarea.

The snapshots have made implementing istreams slightly more complicated than earlier. There are a few different ways to implement istreams:

  • Always point buffer=w_buffer and use i_stream_try_alloc() and/or i_stream_alloc() to allocate the w_buffer. The generic code will handle all the snapshotting. Use i_stream_read_memarea() to read data from parent stream so multiple snapshots aren’t unnecessarily created.

  • Guarantee that if read() returns <=0, the existing buffer will stay valid. Use ISTREAM_CREATE_FLAG_NOOP_SNAPSHOT flag in i_stream_create() so your filter stream isn’t unnecessarily snapshotted (or causing a panic due to missing snapshot() implementation).

    • One way of doing this with filter streams is to read from the parent stream via i_stream_read(parent) and always use buffer=i_stream_get_data(parent). The parent’s snapshotting guarantees that the buffer will stay valid.

  • Implement the snapshot() yourself in the stream. You’ll need to create a new memarea of the current data available via i_stream_get_data() and it must not change, i.e. most likely you’ll need to duplicate the allocated memory. Create a new struct istream_snapshot and assign the allocated memarea to its old_memarea. Fill prev_snapshot field and return your new snapshot. The snapshot will be freed by the generic istream code either when the next read() returns >0 or when the istream is destroyed.

    • See src/lib-mail/istream-header-filter.c or src/lib-dcrypt/istream-decrypt.c for examples of how to do this.

  • Filter streams that only pass through parent stream’s contents without changes can just point to the parent stream. The default snapshotting causes the parent to be snapshotted, so the filter stream can simply use i_stream_read_memarea() and point to the parent’s buffer.

When Dovecot is configured with --enable-devel-checks, i_stream_read() will verify that the first and the last two bytes of the buffer didn’t unexpectedly change due to a read(). While developing istream changes you should use this to make sure the istream is working properly. Running the istream unit test also via valgrind can also be used to verify that the buffer wasn’t freed.