2 commits

Author	SHA1	Message	Date
unknown	c01c4cc359	Bug#19194 (Right recursion in parser for CASE causes excessive stack usage, ... limitation) Bug#24854 (Mixing Searched Case with Simple Case inside Stored Procedure crashes Mysqld) Implemented code review (19194) comments mysql-test/r/sp_stress_case.result: Implemented code review comments : use SQL instead of a shell script to generate the code mysql-test/t/sp_stress_case.test: Adjusted sql/sql_yacc.yy: Added more explicit comments BitKeeper/deleted/.del-sp_stress_case.sh: Delete: mysql-test/t/sp_stress_case.sh	2006-12-11 16:59:02 -07:00
unknown	476eaae84d	Bug#19194 (Right recursion in parser for CASE causes excessive stack usage, ... limitation) Note to the reviewer ==================== Warning: reviewing this patch is somewhat involved. Due to the nature of several issues all affecting the same area, fixing separately each issue is not practical, since each fix can not be implemented and tested independently. In particular, the issues with - rule recursion - nested case statements - forward jump resolution (backpatch list) are tightly coupled (see below). Definitions =========== The expression CASE expr WHEN expr THEN expr WHEN expr THEN expr ... END is a "Simple Case Expression". The expression CASE WHEN expr THEN expr WHEN expr THEN expr ... END is a "Searched Case Expression". The statement CASE expr WHEN expr THEN stmts WHEN expr THEN stmts ... END CASE is a "Simple Case Statement". The statement CASE WHEN expr THEN stmts WHEN expr THEN stmts ... END CASE is a "Searched Case Statement". A "Left Recursive" rule is like list: element | list element ; A "Right Recursive" rule is like list: element | element list ; Left and right recursion produces the same language, the difference only affects the *order* in which the text is parsed. In a descendant parser (usually written manually), right recursion works very well, and is typically implemented with a while loop. In an ascendant parser (yacc/bison) left recursion works very well, and is implemented naturally by the parser stack. In both cases, using the wrong type or recursion is very bad and should be avoided, as it causes technical issues with the parser implementation. Before this change ================== The "Simple Case Expression" and "Searched Case Expression" were both implemented by the "when_list" and "when_list2" rules, which are left recursive (ok). These rules, however, used lex->when_list instead of using the parser stack, which is more complex that necessary, and potentially dangerous because of other rules using THD::reset_lex. The "Simple Case Statement" and "Searched Case Statements" were implemented by the "sp_case", "sp_whens" and in part by "sp_proc_stmt" rules. Both cases were right recursive (bad). The grammar involved was convoluted, and is assumed to be the results of tweaks to get the code generation to work, but is not what someone would naturally write. In addition, using a common rule for both "Simple" and "Searched" case statements was implemented with sp_head::m_flags |= IN_SIMPLE_CASE, which is a flag and not a stack, and therefore does not take into account *nested* case statements. This leads to incorrect generated code, and either a server crash or an incorrect result. With regards to the backpatch mechanism, a *different* backpatch list was created for each jump from "WHEN expr THEN stmt" to "END CASE", which relied on the grammar to be right recursive. This is a mis-use of the backpatch list, since this list can resolve multiple references to the same target at once. The optimizer algorithm used to detect dead code in the "assembly" SQL instructions, implemented by sp_head::opt_mark(uint ip), was recursive in some cases (a conditional jump pointing forward to another conditional jump). In case of specially crafted code, like - a long list of "IF expr THEN stmt END IF" - a long CASE statement this would actually cause a server crash with a stack overflow. In general, having a stack that grows proportionally with user data (the SQL code given by the client in a CREATE PROCEDURE) is to be avoided. In debug builds only, creating a SP / SF / Trigger which had a significant amount of code would spend --literally-- several minutes in sp_head::create, because of the debug code involved with DBUG_PRINT("info", ("Code %s ... There are several issues with this code: - in a CASE with 5 000 WHEN, there are 15 000 instructions generated, which create a sting representation of the code which is 500 000 bytes long, - using a String instead of an io stream causes performances to degrade to a total server freeze, as time is spent doing realloc of a buffer always too short, - Printing a 500 000 long string in the debug log is too verbose, - Generating this string even when DBUG_PRINT is off is useless, - Having code that potentially can affect the server behavior, used with #ifdef / #endif is useful in some cases, but is also a bad practice. After this change ================= "Case Expressions" (both simple and searched) have been simplified to not use LEX::when_list, which has been removed. Considering all the issues affecting case statements, the grammar for these has been totally re written. The existing actions, used to generate "assembly" sp_inst* code, have been preserved but moved in the new grammar, with the following changes: a) Bison rules are no longer shared between "Simple" and "Searched" case statements, because a stack instead of a flag is required to handle them. Nested statements are handled naturally by the parser stack, which by definition uses the correct rule in the correct context. Nested statements of the opposite type (simple vs searched) works correctly. The flag sp_head::IN_SIMPLE_CASE is no longer used. This is a step towards resolution of WL#2999, which correctly identified that temporary parsing flags do not belong to sp_head. The code in the action is shared by mean of the case_stmt_action_xxx() helpers. b) The backpatch mechanism, used to resolve forward jumps in the generated code, has been changed to: - create a label for the instruction following 'END CASE', - register each jump at the end of a "WHEN expr THEN stmt" in a *unique* backpatch list associated with the 'END CASE' label - resolve all the forward jumps for this label at once. In addition, the code involving backpatch has been commented, so that a reader can now understand by reading matching "Registering" and "Resolving" comments how the forward jumps are resolved and what target they resolve to, as this is far from evident when reading the code alone. The implementation of sp_head::opt_mark() has been revised to avoid recursive calls from jump instructions, and instead add the jump location to the list of paths to explore during the flow analysis of the instruction graph, with a call to sp_head::add_mark_lead(). In addition, the flow analysis will stop if an instruction has already been marked as reachable, which the previous code failed to do in the recursive case. sp_head::opt_mark() is now private, to prevent new calls to this method from being introduced. The debug code present in sp_head::create() has been removed. Considering that SHOW PROCEDURE CODE is also available in debug builds, and can be used anytime regardless of the trace level, as opposed to "CREATE PROCEDURE" time and only if the trace was on, removing the code actually makes debugging easier (usable trace). Tests have been written to cover the parser overflow (big CASE), and to cover nested CASE statements. mysql-test/r/sp-code.result: Test cases for nested CASE statements. mysql-test/t/sp-code.test: Test cases for nested CASE statements. sql/sp_head.cc: Re factored opt_mark() to avoid recursion, clean up. sql/sp_head.h: Re factored opt_mark() to avoid recursion, clean up. sql/sql_lex.cc: Removed when_list. sql/sql_lex.h: Removed when_list. sql/sql_yacc.yy: Minor clean up for case expressions, Major re write for case statements (Bug#19194). mysql-test/r/sp_stress_case.result: New test for massive CASE statements. mysql-test/t/sp_stress_case.sh: New test for massive CASE statements. mysql-test/t/sp_stress_case.test: New test for massive CASE statements.	2006-11-17 12:14:29 -07:00