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Commit47c0f00

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Be more wary about NULL values for GUC string variables.
get_explain_guc_options() crashed if a string GUC marked GUC_EXPLAINhas a NULL boot_val. Nosing around found a couple of other placesthat seemed insufficiently cautious about NULL string values, althoughthose are likely unreachable in practice. Add some commentarydefining the expectations for NULL values of string variables,in hopes of forestalling future additions of more such bugs.Xing Guo, Aleksander Alekseev, Tom LaneDiscussion:https://postgr.es/m/CACpMh+AyDx5YUpPaAgzVwC1d8zfOL4JoD-uyFDnNSa1z0EsDQQ@mail.gmail.com
1 parent52a105e commit47c0f00

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2 files changed

+20
-2
lines changed

2 files changed

+20
-2
lines changed

‎src/backend/utils/misc/guc.c

Lines changed: 10 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -9522,7 +9522,14 @@ get_explain_guc_options(int *num)
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{
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structconfig_string*lconf= (structconfig_string*)conf;
95249524

9525-
modified= (strcmp(lconf->boot_val,*(lconf->variable))!=0);
9525+
if (lconf->boot_val==NULL&&
9526+
*lconf->variable==NULL)
9527+
modified= false;
9528+
elseif (lconf->boot_val==NULL||
9529+
*lconf->variable==NULL)
9530+
modified= true;
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else
9532+
modified= (strcmp(lconf->boot_val,*(lconf->variable))!=0);
95269533
}
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break;
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@@ -10269,7 +10276,8 @@ write_one_nondefault_variable(FILE *fp, struct config_generic *gconf)
1026910276
{
1027010277
structconfig_string*conf= (structconfig_string*)gconf;
1027110278

10272-
fprintf(fp,"%s",*conf->variable);
10279+
if (*conf->variable)
10280+
fprintf(fp,"%s",*conf->variable);
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}
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break;
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‎src/include/utils/guc_tables.h

Lines changed: 10 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -218,6 +218,16 @@ struct config_real
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void*reset_extra;
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};
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221+
/*
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* A note about string GUCs: the boot_val is allowed to be NULL, which leads
223+
* to the reset_val and the actual variable value (*variable) also being NULL.
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* However, there is no way to set a NULL value subsequently using
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* set_config_option or any other GUC API. Also, GUC APIs such as SHOW will
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* display a NULL value as an empty string. Callers that choose to use a NULL
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* boot_val should overwrite the setting later in startup, or else be careful
228+
* that NULL doesn't have semantics that are visibly different from an empty
229+
* string.
230+
*/
221231
structconfig_string
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{
223233
structconfig_genericgen;

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