PQcancelCreate #Prepares a connection over which a cancel request can be sent.
PGcancelConn *PQcancelCreate(PGconn *conn);
PQcancelCreate creates a
PGcancelConn
object, but it won't instantly start sending a cancel request over this
connection. A cancel request can be sent over this connection in a
blocking manner using PQcancelBlocking and in a
non-blocking manner using PQcancelStart.
The return value can be passed to PQcancelStatus
to check if the PGcancelConn object was
created successfully. The PGcancelConn object
is an opaque structure that is not meant to be accessed directly by the
application. This PGcancelConn object can be
used to cancel the query that's running on the original connection in a
thread-safe way.
Many connection parameters of the original client will be reused when
setting up the connection for the cancel request. Importantly, if the
original connection requires encryption of the connection and/or
verification of the target host (using sslmode or
gssencmode), then the connection for the cancel
request is made with these same requirements. Any connection options
that are only used during authentication or after authentication of the
client are ignored though, because cancellation requests do not require
authentication and the connection is closed right after the cancellation
request is submitted.
Note that when PQcancelCreate returns a non-null
pointer, you must call PQcancelFinish when you
are finished with it, in order to dispose of the structure and any
associated memory blocks. This must be done even if the cancel request
failed or was abandoned.
PQcancelBlocking #Requests that the server abandons processing of the current command in a blocking manner.
int PQcancelBlocking(PGcancelConn *cancelConn);
The request is made over the given PGcancelConn,
which needs to be created with PQcancelCreate.
The return value of PQcancelBlocking
is 1 if the cancel request was successfully
dispatched and 0 if not. If it was unsuccessful, the error message can be
retrieved using
PQcancelErrorMessage
.
Successful dispatch of the cancellation is no guarantee that the request will have any effect, however. If the cancellation is effective, the command being canceled will terminate early and return an error result. If the cancellation fails (say, because the server was already done processing the command), then there will be no visible result at all.
PQcancelStartPQcancelPoll #Requests that the server abandons processing of the current command in a non-blocking manner.
int PQcancelStart(PGcancelConn *cancelConn); PostgresPollingStatusType PQcancelPoll(PGcancelConn *cancelConn);
The request is made over the given PGcancelConn,
which needs to be created with PQcancelCreate.
The return value of PQcancelStart
is 1 if the cancellation request could be started and 0 if not.
If it was unsuccessful, the error message can be
retrieved using
PQcancelErrorMessage
.
If PQcancelStart succeeds, the next stage
is to poll libpq so that it can proceed with
the cancel connection sequence.
Use PQcancelSocket to obtain the descriptor of the
socket underlying the database connection.
(Caution: do not assume that the socket remains the same
across PQcancelPoll calls.)
Loop thus: If PQcancelPoll(cancelConn) last returned
PGRES_POLLING_READING, wait until the socket is ready to
read (as indicated by select(),
poll(), or similar system function).
Then call PQcancelPoll(cancelConn) again.
Conversely, if PQcancelPoll(cancelConn) last returned
PGRES_POLLING_WRITING, wait until the socket is ready
to write, then call PQcancelPoll(cancelConn) again.
On the first iteration, i.e., if you have yet to call
PQcancelPoll(cancelConn), behave as if it last returned
PGRES_POLLING_WRITING. Continue this loop until
PQcancelPoll(cancelConn) returns
PGRES_POLLING_FAILED, indicating the connection procedure
has failed, or PGRES_POLLING_OK, indicating cancel
request was successfully dispatched.
Successful dispatch of the cancellation is no guarantee that the request will have any effect, however. If the cancellation is effective, the command being canceled will terminate early and return an error result. If the cancellation fails (say, because the server was already done processing the command), then there will be no visible result at all.
At any time during connection, the status of the connection can be
checked by calling PQcancelStatus.
If this call returns CONNECTION_BAD, then
the cancel procedure has failed; if the call returns
CONNECTION_OK, then cancel request was
successfully dispatched.
Both of these states are equally detectable from the return value of
PQcancelPoll, described above.
Other states might also occur during (and only during) an asynchronous
connection procedure.
These indicate the current stage of the connection procedure and might
be useful to provide feedback to the user for example.
These statuses are:
CONNECTION_ALLOCATED #
Waiting for a call to PQcancelStart or
PQcancelBlocking, to actually open the
socket. This is the connection state right after
calling PQcancelCreate
or PQcancelReset. No connection to the
server has been initiated yet at this point. To actually start
sending the cancel request use PQcancelStart or
PQcancelBlocking.
CONNECTION_STARTED #Waiting for connection to be made.
CONNECTION_MADE #Connection OK; waiting to send.
CONNECTION_AWAITING_RESPONSE #Waiting for a response from the server.
CONNECTION_SSL_STARTUP #Negotiating SSL encryption.
CONNECTION_GSS_STARTUP #Negotiating GSS encryption.
Note that, although these constants will remain (in order to maintain compatibility), an application should never rely upon these occurring in a particular order, or at all, or on the status always being one of these documented values. An application might do something like this:
switch(PQcancelStatus(conn))
{
case CONNECTION_STARTED:
feedback = "Connecting...";
break;
case CONNECTION_MADE:
feedback = "Connected to server...";
break;
.
.
.
default:
feedback = "Connecting...";
}
The connect_timeout connection parameter is ignored
when using PQcancelPoll; it is the application's
responsibility to decide whether an excessive amount of time has elapsed.
Otherwise, PQcancelStart followed by a
PQcancelPoll loop is equivalent to
PQcancelBlocking.
PQcancelStatus #Returns the status of the cancel connection.
ConnStatusType PQcancelStatus(const PGcancelConn *cancelConn);
The status can be one of a number of values. However, only three of
these are seen outside of an asynchronous cancel procedure:
CONNECTION_ALLOCATED,
CONNECTION_OK and
CONNECTION_BAD. The initial state of a
PGcancelConn that's successfully created using
PQcancelCreate is CONNECTION_ALLOCATED.
A cancel request that was successfully dispatched
has the status CONNECTION_OK. A failed
cancel attempt is signaled by status
CONNECTION_BAD. An OK status will
remain so until PQcancelFinish or
PQcancelReset is called.
See the entry for PQcancelStart with regards
to other status codes that might be returned.
Successful dispatch of the cancellation is no guarantee that the request will have any effect, however. If the cancellation is effective, the command being canceled will terminate early and return an error result. If the cancellation fails (say, because the server was already done processing the command), then there will be no visible result at all.
PQcancelSocket #Obtains the file descriptor number of the cancel connection socket to the server.
int PQcancelSocket(const PGcancelConn *cancelConn);
A valid descriptor will be greater than or equal to 0;
a result of -1 indicates that no server connection is currently open.
This might change as a result of calling any of the functions
in this section on the PGcancelConn
(except for
PQcancelErrorMessage
and
PQcancelSocket itself).
PQcancelErrorMessage
#Returns the error message most recently generated by an operation on the cancel connection.
char *PQcancelErrorMessage(const PGcancelConn *cancelconn);
Nearly all libpq functions that take a
PGcancelConn will set a message for
PQcancelErrorMessage
if they fail.
Note that by libpq convention,
a nonempty
PQcancelErrorMessage
result
can consist of multiple lines, and will include a trailing newline.
The caller should not free the result directly.
It will be freed when the associated
PGcancelConn handle is passed to
PQcancelFinish. The result string should not be
expected to remain the same across operations on the
PGcancelConn structure.
PQcancelFinish #
Closes the cancel connection (if it did not finish sending the
cancel request yet). Also frees memory used by the
PGcancelConn object.
void PQcancelFinish(PGcancelConn *cancelConn);
Note that even if the cancel attempt fails (as
indicated by PQcancelStatus), the
application should call PQcancelFinish
to free the memory used by the PGcancelConn
object.
The PGcancelConn pointer must not be used
again after PQcancelFinish has been called.
PQcancelReset #
Resets the PGcancelConn so it can be reused for a new
cancel connection.
void PQcancelReset(PGcancelConn *cancelConn);
If the PGcancelConn is currently used to send a cancel
request, then this connection is closed. It will then prepare the
PGcancelConn object such that it can be used to send a
new cancel request.
This can be used to create one PGcancelConn
for a PGconn and reuse it multiple times
throughout the lifetime of the original PGconn.
These functions represent older methods of sending cancel requests.
Although they still work, they are deprecated due to not sending the cancel
requests in an encrypted manner, even when the original connection
specified sslmode or gssencmode to
require encryption. Thus these older methods are heavily discouraged from
being used in new code, and it is recommended to change existing code to
use the new functions instead.
PQgetCancel #
Creates a data structure containing the information needed to cancel
a command using PQcancel.
PGcancel *PQgetCancel(PGconn *conn);
PQgetCancel creates a
PGcancel
object given a PGconn connection object.
It will return NULL if the given conn
is NULL or an invalid connection.
The PGcancel object is an opaque
structure that is not meant to be accessed directly by the
application; it can only be passed to PQcancel
or PQfreeCancel.
PQfreeCancel #
Frees a data structure created by PQgetCancel.
void PQfreeCancel(PGcancel *cancel);
PQfreeCancel frees a data object previously created
by PQgetCancel.
PQcancel #
PQcancel is a deprecated and insecure
variant of PQcancelBlocking, but one that can be
used safely from within a signal handler.
int PQcancel(PGcancel *cancel, char *errbuf, int errbufsize);
PQcancel only exists because of backwards
compatibility reasons. PQcancelBlocking should be
used instead. The only benefit that PQcancel has
is that it can be safely invoked from a signal handler, if the
errbuf is a local variable in the signal handler.
However, this is generally not considered a big enough benefit to be
worth the security issues that this function has.
The PGcancel object is read-only as far as
PQcancel is concerned, so it can also be invoked
from a thread that is separate from the one manipulating the
PGconn object.
The return value of PQcancel is 1 if the
cancel request was successfully dispatched and 0 if not.
If not, errbuf is filled with an explanatory
error message.
errbuf must be a char array of size
errbufsize (the recommended size is 256 bytes).
PQrequestCancel #
PQrequestCancel is a deprecated and insecure
variant of PQcancelBlocking.
int PQrequestCancel(PGconn *conn);
PQrequestCancel only exists because of backwards
compatibility reasons. PQcancelBlocking should be
used instead. There is no benefit to using
PQrequestCancel over
PQcancelBlocking.
Requests that the server abandon processing of the current
command. It operates directly on the
PGconn object, and in case of failure stores the
error message in the PGconn object (whence it can
be retrieved by
PQerrorMessage
). Although
the functionality is the same, this approach is not safe within
multiple-thread programs or signal handlers, since it is possible
that overwriting the PGconn's error message will
mess up the operation currently in progress on the connection.