## 背景 我们执行一个大SQL时(长度大于512M),会返回如下错误: ~~~ ERROR: invalid memory alloc request size 1073741824 ~~~ ## 复现 我们首先复现出来这个问题 1. 创建表 ~~~ create table byteatable(id int, obj bytea); ~~~ 2. 插入512M大对象 ~~~ #!/bin/bash data='a' for ((i=1;i<=29;i++)); do data=$data$data done echo 'build ok' psql -U postgres -d postgres << EOF insert into byteatable(id,obj) values (1,"$data"); EOF echo 'OK' ~~~ 执行脚本后就能复现出来了 ~~~ ERROR: invalid memory alloc request size 1073741824 ~~~ ## BUG分析 我们先找到出现这个错误的位置。 源码位置: ~~~ void * MemoryContextAlloc(MemoryContext context, Size size) { void *ret; AssertArg(MemoryContextIsValid(context)); if (!AllocSizeIsValid(size)) elog(ERROR, "invalid memory alloc request size %zu", size); context->isReset = false; ret = (*context->methods->alloc) (context, size); VALGRIND_MEMPOOL_ALLOC(context, ret, size); return ret; } #define AllocSizeIsValid(size) ((Size) (size) <= MaxAllocSize) #define MaxAllocSize ((Size) 0x3fffffff) /* 1 gigabyte - 1 */ ~~~ 这里限制的内存是1G - 1,而我们插入的大SQL需要的内存没有1G,那么为什么还提示非法的申请内存大小呢?通过调试跟踪,我们发现是在词法分析的时刻出错的。主要问题是在词法分析的内存申请机制上,申请的内存肯定不能少于sql的长度,如果当前申请的内存不够用,那么将重新申请当前内存乘以2的内存大小。 ~~~ int literallen; /* actual current string length */ int literalalloc; /* current allocated buffer size */ //当前申请内存初始化 yyext->literalalloc = 1024; ~~~ ~~~ static void addlit(char *ytext, int yleng, core_yyscan_t yyscanner) { /* enlarge buffer if needed */ if ((yyextra->literallen + yleng) >= yyextra->literalalloc) { do { yyextra->literalalloc *= 2; } while ((yyextra->literallen + yleng) >= yyextra->literalalloc); yyextra->literalbuf = (char *) repalloc(yyextra->literalbuf, yyextra->literalalloc); } /* append new data */ memcpy(yyextra->literalbuf + yyextra->literallen, ytext, yleng); yyextra->literallen += yleng; } static void addlitchar(unsigned char ychar, core_yyscan_t yyscanner) { /* enlarge buffer if needed */ if ((yyextra->literallen + 1) >= yyextra->literalalloc) { yyextra->literalalloc *= 2; yyextra->literalbuf = (char *) repalloc(yyextra->literalbuf, yyextra->literalalloc); } } ~~~ 从源码中可以看出,每次申请原申请内存的2倍,即yyextra->literalalloc *= 2; 而最大申请内存限制是 ~~~ #define MaxAllocSize ((Size) 0x3fffffff) /* 1 gigabyte - 1 */ ~~~ 所以我们在词法分析能申请的最大内存是2^29 = 536870912,如果词法分析SQL语句需要的内存大于536870912,那么申请的内存需要再乘2,就会得到2^30 = 1073741824,超过MaxAllocSize=0x3fffffff= 1073741823。所以会提示错误: ~~~ ERROR: invalid memory alloc request size 1073741824 ~~~ 当然不仅仅是插入一个大对象才会引起这个问题,只要是SQL语句长度大于512M都是出现这个错误,我们可以使用select复现: ~~~ do language plpgsql $$ declare v_text text := 'a'; begin for i in 1..29 loop v_text:=v_text||v_text; end loop; execute $_$select '$_$||v_text||$_$'$_$; raise notice 'execute a sql large than 512MB success.'; exception when others then raise notice 'execute a sql large than 512MB failed.'; end; $$; ~~~ ## BUG修复 其实申请MaxAllocSize是可行的,通过修改源码实现,每当申请的内存大于MaxAllocSize并且SQL需要的长度小于MaxAllocSize时,我们就申请MaxAllocSize大小的内存。 ~~~ static void addlit(char *ytext, int yleng, core_yyscan_t yyscanner) { /* enlarge buffer if needed */ if ((yyextra->literallen + yleng) >= yyextra->literalalloc) { do { yyextra->literalalloc *= 2; } while ((yyextra->literallen + yleng) >= yyextra->literalalloc); /* we can not alloc more than MaxAllocSize */ if (yyextra->literalalloc > MaxAllocSize && (yyextra->literallen + yleng) < MaxAllocSize) yyextra->literalalloc = MaxAllocSize; yyextra->literalbuf = (char *) repalloc(yyextra->literalbuf, yyextra->literalalloc); } /* append new data */ memcpy(yyextra->literalbuf + yyextra->literallen, ytext, yleng); yyextra->literallen += yleng; } ~~~ ~~~ static void addlitchar(unsigned char ychar, core_yyscan_t yyscanner) { /* enlarge buffer if needed */ if ((yyextra->literallen + 1) >= yyextra->literalalloc) { yyextra->literalalloc *= 2; /* we can not alloc more than MaxAllocSize */ if (yyextra->literalalloc > MaxAllocSize && (yyextra->literallen + 1) < MaxAllocSize) yyextra->literalalloc = MaxAllocSize; yyextra->literalbuf = (char *) repalloc(yyextra->literalbuf, yyextra->literalalloc); } /* append new data */ yyextra->literalbuf[yyextra->literallen] = ychar; yyextra->literallen += 1; } ~~~ 修复之后可以正常插入了 ~~~ $ sh pgtest.sh build ok INSERT 0 1 OK ~~~