first commit, SM2-DSA only

doc/crypto/BIO_should_retry.pod

@@ -0,0 +1,114 @@
+=pod
+
+=head1 NAME
+
+BIO_should_retry, BIO_should_read, BIO_should_write,
+BIO_should_io_special, BIO_retry_type, BIO_should_retry,
+BIO_get_retry_BIO, BIO_get_retry_reason - BIO retry functions
+
+=head1 SYNOPSIS
+
+ #include <openssl/bio.h>
+
+ #define BIO_should_read(a)		((a)->flags & BIO_FLAGS_READ)
+ #define BIO_should_write(a)		((a)->flags & BIO_FLAGS_WRITE)
+ #define BIO_should_io_special(a)	((a)->flags & BIO_FLAGS_IO_SPECIAL)
+ #define BIO_retry_type(a)		((a)->flags & BIO_FLAGS_RWS)
+ #define BIO_should_retry(a)		((a)->flags & BIO_FLAGS_SHOULD_RETRY)
+
+ #define BIO_FLAGS_READ		0x01
+ #define BIO_FLAGS_WRITE	0x02
+ #define BIO_FLAGS_IO_SPECIAL	0x04
+ #define BIO_FLAGS_RWS (BIO_FLAGS_READ|BIO_FLAGS_WRITE|BIO_FLAGS_IO_SPECIAL)
+ #define BIO_FLAGS_SHOULD_RETRY	0x08
+
+ BIO *	BIO_get_retry_BIO(BIO *bio, int *reason);
+ int	BIO_get_retry_reason(BIO *bio);
+
+=head1 DESCRIPTION
+
+These functions determine why a BIO is not able to read or write data.
+They will typically be called after a failed BIO_read() or BIO_write()
+call.
+
+BIO_should_retry() is true if the call that produced this condition
+should then be retried at a later time.
+
+If BIO_should_retry() is false then the cause is an error condition.
+
+BIO_should_read() is true if the cause of the condition is that a BIO
+needs to read data.
+
+BIO_should_write() is true if the cause of the condition is that a BIO
+needs to read data.
+
+BIO_should_io_special() is true if some "special" condition, that is a
+reason other than reading or writing is the cause of the condition.
+
+BIO_retry_type() returns a mask of the cause of a retry condition
+consisting of the values B<BIO_FLAGS_READ>, B<BIO_FLAGS_WRITE>,
+B<BIO_FLAGS_IO_SPECIAL> though current BIO types will only set one of
+these.
+
+BIO_get_retry_BIO() determines the precise reason for the special
+condition, it returns the BIO that caused this condition and if 
+B<reason> is not NULL it contains the reason code. The meaning of
+the reason code and the action that should be taken depends on
+the type of BIO that resulted in this condition.
+
+BIO_get_retry_reason() returns the reason for a special condition if
+passed the relevant BIO, for example as returned by BIO_get_retry_BIO().
+
+=head1 NOTES
+
+If BIO_should_retry() returns false then the precise "error condition"
+depends on the BIO type that caused it and the return code of the BIO
+operation. For example if a call to BIO_read() on a socket BIO returns
+0 and BIO_should_retry() is false then the cause will be that the
+connection closed. A similar condition on a file BIO will mean that it
+has reached EOF. Some BIO types may place additional information on
+the error queue. For more details see the individual BIO type manual
+pages.
+
+If the underlying I/O structure is in a blocking mode almost all current
+BIO types will not request a retry, because the underlying I/O
+calls will not. If the application knows that the BIO type will never
+signal a retry then it need not call BIO_should_retry() after a failed
+BIO I/O call. This is typically done with file BIOs.
+
+SSL BIOs are the only current exception to this rule: they can request a
+retry even if the underlying I/O structure is blocking, if a handshake
+occurs during a call to BIO_read(). An application can retry the failed
+call immediately or avoid this situation by setting SSL_MODE_AUTO_RETRY
+on the underlying SSL structure.
+
+While an application may retry a failed non blocking call immediately
+this is likely to be very inefficient because the call will fail
+repeatedly until data can be processed or is available. An application
+will normally wait until the necessary condition is satisfied. How
+this is done depends on the underlying I/O structure.
+
+For example if the cause is ultimately a socket and BIO_should_read()
+is true then a call to select() may be made to wait until data is
+available and then retry the BIO operation. By combining the retry
+conditions of several non blocking BIOs in a single select() call
+it is possible to service several BIOs in a single thread, though
+the performance may be poor if SSL BIOs are present because long delays
+can occur during the initial handshake process. 
+
+It is possible for a BIO to block indefinitely if the underlying I/O
+structure cannot process or return any data. This depends on the behaviour of
+the platforms I/O functions. This is often not desirable: one solution
+is to use non blocking I/O and use a timeout on the select() (or
+equivalent) call.
+
+=head1 BUGS
+
+The OpenSSL ASN1 functions cannot gracefully deal with non blocking I/O:
+that is they cannot retry after a partial read or write. This is usually
+worked around by only passing the relevant data to ASN1 functions when
+the entire structure can be read or written.
+
+=head1 SEE ALSO
+
+TBA