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.
* mmap: Read data from file using 'mmap()'. This usually seems to be slower
than just using it with 'read()', so this input stream is probably quite
unnecessary.
* 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-mail/dot: Read SMTP-style DATA input where the input ends with an empty
"." line.
* lib-mail/header-filter: Add/remove/modify email headers.
* lib-compression/*: Read zlib/bzlib/lz4/lzma compressed data.
Reading
-------
'i_stream_read()' tries to read more data into the stream's buffer. It returns:
* -2: Nothing was read, because 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 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 usually given as parameter to the 'i_stream_create_*()', filters 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.
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.
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, buffer size has no limits */
struct istream *input = i_stream_create_fd(file_fd, (size_t)-1, 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-internal.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.
* '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 more more details).
There are some variables available:
* 'buffer' contains pointer to the data.
* 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_w_buffer_realloc(old_size)' reallocates 'w_buffer' to the
current 'buffer_size'. If memarea's refcount is 1, this can be done with
'i_realloc()', otherwise new memory is allocated.
* '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 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.
* 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.
(This file was created from the wiki on 2019-06-19 12:42)
