babelfish-for-postgresql / babelfish_extensions / 8434518778

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Send env change token in tds when error abort transaction block (#2401)

In drivers which manage transaction automatically for the user, it is necessary to send the ENVCHANGE token on error which aborts a user transaction block. Other wise the driver will not be aware of this TRANSACTION ABORT.
For example consider this pyodbc case: (autocommit disabled, which also the default value in pyodbc)

cursor = cnxn.cursor()
cursor.execute("insert into t values (1)")
cursor.execute("insert into t values (2)")
try:
    cursor.execute("insert into t values ('aaaa')")
except:
    print("Let execution continue")
cursor.execute("insert into t values (3)")
cursor.execute("insert into t values (4)")
cnxn.rollback()
This currently gets executed as

BEGIN TRAN -- driver managed
INSERT INTO t VALUES (1)
INSERT INTO t VALUES (2)
INSERT INTO t VALUES ('aaaa') -- We hit an error here -- txn block aborted
INSERT INTO t VALUES (3)  -- No active transaction block -- auto commit behaviour
INSERT INTO t VALUES (4)  -- No active transaction block -- auto commit behaviour
ROLLBACK -- No transaction block active, we will get error
table 't' will contain values 3,4. Ideally this should have been empty since we are not committing any txn.
If we had notified driver about the transaction rollback on error, the execution would look like

BEGIN TRAN -- driver managed
INSERT INTO t VALUES (1)
INSERT INTO t VALUES (2)
INSERT INTO t VALUES ('aaaa') -- We hit an error here -- txn block aborted & driver notified
BEGIN TRAN -- since we notified the driver about the rollback
INSERT INTO t VALUES (3)  -- Active txn block, not yet committed
INSERT INTO t VALUES (4)  -- Active txn block, not yet committed
ROLLBACK -- txn block aborted
table 't' empty

Issues Resolved
[BABEL-4797]

Signed-off-by: Tanzeel Khan <tzlkhan@amazon.com>

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