32.7. Canceling Queries in Progress

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.

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.

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:

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 .

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.

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).

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.

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.

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 .

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 .

Frees a data structure created by PQgetCancel .

void PQfreeCancel(PGcancel *cancel);

PQfreeCancel frees a data object previously created by PQgetCancel .

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 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.