gdb: unbuffer all input streams when not using readline

gdb: unbuffer all input streams when not using readline

This commit should fix PR gdb/28711.  What's actually going on is
pretty involved, and there's still a bit of the story that I don't
understand completely, however, from my observed results, I think that
the change I propose making here (or something very similar) is going
to be needed.

The original bug report involves using eclipse to drive gdb using mi
commands.  A separate tty is spun off in which to send gdb the mi
commands, this tty is created using the new-ui command.

The behaviour observed is that, given a particular set of mi commands
being sent to gdb, we sometimes see an ESPIPE error from a lseek
call, which ultimately results in gdb terminating.

The problems all originate from gdb_readline_no_editing_callback in
gdb/event-top.c, where we can (sometimes) perform calls to fgetc, and
allow glibc to perform buffering on the FILE object being used.

I say sometime, because, gdb_readline_no_editing_callback already
includes a call to disable the glibc buffering, but this is only done
if the input stream is not a tty.  In our case the input stream is a
tty, so the buffering is left in place.

The first step to understanding why this problem occurs is to
understand that eclipse sends multiple commands to gdb very quickly
without waiting for and answer to each command, eclipse plans to
collect all of the command results after sending all the commands to
gdb.  In fact, eclipse sends the commands to gdb that they appear to
arrive in the gdb process as a single block of data.  When reproducing
this issue within the testsuite I find it necessary to send multiple
commands using a single write call.

The next bit of the story gets a little involved, and this is where my
understanding is not complete.  I can describe the behaviour that I
observe, and (for me at least) I'm happy that what I'm seeing, if a
little strange, is consistent.  In order to fully understand what's
going on I think I would likely need to dive into kernel code, which
currently seems unnecessary given that I'm happy with the solution I'm
proposing.

The following description all relates to input from a tty in which I'm
not using readline.  I see the same problems either when using a
new-ui tty, or with gdb's standard, non-readline, mi tty.

Here's what I observe happening when I send multiple commands to gdb
using a single write, if I send gdb this:

  command_1\ncommand_2\ncommand_3

then gdb's event loop will wake up (from its select) as it sees there
is input available.  We call into gdb_readline_no_editing_callback,
where we call fgetc, glibc will do a single big read, and get back
just:

  command_1\n

that is, despite there being multiple lines of input available, I
consistently get just a single line.  From glibc a single character is
returned from the fgetc call, and within gdb we accumulate characters,
one at a time, into our own buffer.  Eventually gdb sees the '\n'
character, and dispatches the whole 'command_1' into gdb's command
handler, which processes the command and prints the result.  We then
return to gdb_readline_no_editing_callback, which in turn returns to
gdb's event loop where we re-enter the select.

Inside the select we immediately see that there is more input waiting
on the input stream, drop out of the select, and call back into
gdb_readline_no_editing_callback.  In this function we again call
fgetc where glibc performs another big read.  This time glibc gets:

  command_2\n

that is, we once again get just a single line, despite there being a
third line available.  Just like the first command we copy the whole
string, character by character into gdb's buffer, then handle the
command.  After handling the command we go to the event loop, enter,
and then exit the select, and call back to the function
gdb_readline_no_editing_callback.

In gdb_readline_no_editing_callback we again call fgetc, this time
glibc gets the string:

  command_3\n

like before, we copy this to gdb's buffer and handle the command, then
we return to the event loop.  At this point the select blocks while we
wait for more input to arrive.

The important bit of this is that someone, somewhere is, it appears,
taking care to split the incoming write into lines.

My next experiment is to try something like:

  this_is_a_very_long_command\nshort_command\n

However, I actually make 'this_is_a_very_long_command' very long, as
in many hundreds of characters long.  One way to do this is:

  echo xxxxxx.....xxxxx

and just adding more and more 'x' characters as needed.  What I'm
aiming for is to have the first command be longer than glibc's
internal read buffer, which, on my machine, is 1024 characters.

However, for this discussion, lets imagine that glibc's buffer is just
8 characters (we can create just this situation by adding a suitable
setbuf call into gdb_readline_no_editing_callback).

Now, if I send gdb this data:

  abcdefghij\nkl\n

The first read from glibc will get 'abcdefgh', that is a full 8
character buffer.  Once gdb has copied these to its buffer we call
fgetc again, and now glibc will get 'ij\n', that is, just like before,
multiple lines are split at the '\n' character.  The full command,
which is now in gdb's buffer can be handled 'abcdefghij', after which
we go (via the event loop) back to gdb_readline_no_editing_callback.
Now we call fgetc, and glibc will get 'kl\n', which is then handled in
the normal way.

So far, so good.  However, there is, apparently, one edge case where
the above rules don't apply.

If the '\n' character is the first character read from the kernel,
then the incoming lines are not split up.  So, given glibc's 8
character buffer, if I send gdb this:

  abcdefgh\nkl\n

that is the first command is 8 characters plus a newline, then, on the
first read (from within glibc) we get 'abcdefgh' in a single buffer.
As there's no newline gdb calls fgetc again, and glibc does another
large read, now we get:

  \nkl\n

which doesn't follow the above pattern - the lines are not split into
separate buffers!

So, gdb reads the first character from glibc using fgetc, this is the
newline.  Now gdb has a complete command, and so the command is
handled.  We then return to the event loop and enter the select.

The problem is that, as far as the kernel is concerned, there is no
more input pending, it's all been read into glibc's buffer, and so the
select doesn't return.  The second command is basically stuck in
glibc's buffer.

If I send another command to gdb, or even just send an empty
command (a lone newline) then the select returns, we call into
gdb_readline_no_editing_callback, and now gdb sees the second
command.

OK, so the above is interesting, but it doesn't explain the ESPIPE
error.

Well, that's a slightly different, but related issue.  The ESPIPE
case will _only_ show up when using new-ui to create the separate tty
for mi commands, and is a consequence of this commit:

  commit afe09f0b6311a4dd1a7e2dc6491550bb228734f8
  Date:   Thu Jul 18 17:20:04 2019 +0100

      Fix for using named pipes on Windows

Prior to this commit, the new-ui command would open the tty three
times, once each for stdin, stderr, and stdout.  After this commit we
open the tty just once and reuse the FILE object for all three roles.

Consider the problem case, where glibc has (unexpectedly) read the
second command into its internal buffer.  When we handle the first
command we usually end up having to write something to the mi output
stream.

After the above commit the same FILE object represents both the input
and output streams, so, when gdb tries to write to the FILE object,
glibc spots that there is input pending within the input buffer, and
so assumes that we have read ahead of where we should be in the input
file.  To correct for this glibc tries to do an lseek call to
reposition the file offset of the output stream prior to writing to
it.  However, as the output stream is a tty, and seeking is not
supported on a tty, this lseek call fails, this results in the ESPIPE,
which ultimately causes gdb to terminate.

So, now we understand why the ESPIPE triggers (which was what caused
the gdb crash in the original bug report), and we also understand that
sometime gdb will not handle the second command in a timely
fashion (if the first command is just the wrong length). So, what to
do about all this?

We could revert the commit mentioned above (and implement its
functionality another way).  This would certainly resolve the ESPIPE
issue, the buffered input would now only be on the input stream, the
output stream would have no buffered input, and so glibc would never
try to lseek, and so we'd never get the ESPIPE error.

However, this only solves one of the two problems.  We would still
suffer from the problem where, if the first command is just the wrong
length, the second command will not (immediately) get handled.

The only solution I can see to this problem is to unbuffer the input
stream.  If glibc is not buffering the input, but instead, we read
incoming data character by character from the kernel, then everything
will be fine.  As soon as we see the newline at the end of the first
command we will handle the first command.  As glibc will have no
buffered input it will not be tempted to lseek, so no ESPIPE error.
When we go have to the event loop there will be more data pending in
the kernel, so the select will immediately return, and the second
command will be processed.

I'm tempted to suggest that we should move the unbuffering of the
input stream out of gdb_readline_no_editing_callback and do it
somewhere earlier, more like when we create the input streams.
However, I've not done that in this commit for a couple of reasons:

  1. By keeping the unbuffering in gdb_readline_no_editing_callback
  I'm making the smallest possible change that fixes the bug.  Moving
  the unbuffering somewhere better can be done as a refactor later, if
  that 's felt to be important,

  2. I don't think making repeated calls to unbuffer the input will
  have that much performance impact.  We only make the unbuffer call
  once per call to gdb_readline_no_editing_callback, and, if the input
  stream is already unbuffered we'll return pretty quickly, so I don't
  see this as being massively costly,

  3. Tom is currently doing lots of gdb stream management changes and
  I want to minimise the chances we'll conflict.

So, this commit just changes gdb_readline_no_editing_callback to
always unbuffer the input stream.

The test for this issue sends two commands in a loop, with the first
command growing bigger each time around the loop.  I actually make the
first command bigger by just adding whitespace to the front, as gdb
still has to read the complete command (including whitespace) via
glibc, so this is enough to trigger the bug.

The original bug was reported when using a virtual machine, and in
this situation we see this in the strace output:

  read(9, "70-var-info-path-expression var1.aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", 1024) = 64
  read(9, "\n71-var-info-path-expression var1.aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\n", 1024) = 67

I'm not completely sure what's going on here, but it appears that the
kernel on the virtual machine is delivering the input to glibc slower
than I see on my real hardware; glibc asks for 1024 bytes, but only
gets 64 bytes the first time.  In the second read we see the problem
case, the first character is the newline, but then the entire second
command is included.

If I run this exact example on my real hardware then the first command
would not be truncated at 64 bytes, instead, I'd expect to see the
newline included in the first read, with the second command split into
a second read.

So, for testing, I check cases where the first command is just a few
characters (starting at 8 character), all the way up to 2048
characters.  Hopefully, this should mean we hit the problem case for
most machine setups.

The only last question relates to commit afe09f0b6311a4d that I
mentioned earlier.  That commit was intended to provide support for
Microsoft named pipes:

  https://docs.microsoft.com/en-us/windows/win32/ipc/named-pipes

I know next to nothing about this topic beyond a brief scan of the
above link, but I think these windows named pipe are closer in
behaviour to unix sockets than to unix named fifos.

I am a little nervous that, after the above commit, we now use the
same FILE for in, err, and out streams.  In contrast, in a vanilla C
program, I would expect different FILE objects for each stream.

Still, I'm reluctant to revert the above commit (and provide the same
functionality a different way) without a specific bug to point at,
and, now that the streams are unbuffered, I expect a lot of the read
and write calls are going straight to the kernel with minimal glibc
involvement, so maybe it doesn't really matter.  Anyway, I haven't
touched the above patch, but it is something to keep in mind when
working in this area.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=28711