I wrote:
> Even granting the premise, the proposed patch seems like a significant
> decrease in user-friendliness for typical cases.  I'd rather see us
> make an effort to print one valid-per-the-DB-encoding character.
> Now that we can rely on snprintf to count %s restrictions in bytes,
> I think something like this should work:
>                          errmsg("\"%.*s\" is not a valid binary digit",
>                                 pg_mblen(sp), sp)));
> But the real problem is that this is only the tip of the iceberg.
> You didn't even hit all the %c usages in varbit.c.

I went through all the %c format sequences in the backend to see which
ones could use this type of fix.  There were not as many as I'd expected,
but still a fair number.  (I skipped cases where the input was coming from
the catalogs, as well as some non-user-facing debug printouts.)  That
leads to the attached patch, which seems to do the job without breaking
anything that works today.

                        regards, tom lane

PS: I failed to resist the temptation to improve some shoddy error
messages nearby in pageinspect/heapfuncs.c.

diff --git a/contrib/hstore/hstore_io.c b/contrib/hstore/hstore_io.c
index 60bdbea46b..b3304ff844 100644
--- a/contrib/hstore/hstore_io.c
+++ b/contrib/hstore/hstore_io.c
@@ -80,7 +80,9 @@ get_val(HSParser *state, bool ignoreeq, bool *escaped)
 			}
 			else if (*(state->ptr) == '=' && !ignoreeq)
 			{
-				elog(ERROR, "Syntax error near '%c' at position %d", *(state->ptr), (int32) (state->ptr - state->begin));
+				elog(ERROR, "Syntax error near \"%.*s\" at position %d",
+					 pg_mblen(state->ptr), state->ptr,
+					 (int32) (state->ptr - state->begin));
 			}
 			else if (*(state->ptr) == '\\')
 			{
@@ -219,7 +221,9 @@ parse_hstore(HSParser *state)
 			}
 			else if (!isspace((unsigned char) *(state->ptr)))
 			{
-				elog(ERROR, "Syntax error near '%c' at position %d", *(state->ptr), (int32) (state->ptr - state->begin));
+				elog(ERROR, "Syntax error near \"%.*s\" at position %d",
+					 pg_mblen(state->ptr), state->ptr,
+					 (int32) (state->ptr - state->begin));
 			}
 		}
 		else if (st == WGT)
@@ -234,7 +238,9 @@ parse_hstore(HSParser *state)
 			}
 			else
 			{
-				elog(ERROR, "Syntax error near '%c' at position %d", *(state->ptr), (int32) (state->ptr - state->begin));
+				elog(ERROR, "Syntax error near \"%.*s\" at position %d",
+					 pg_mblen(state->ptr), state->ptr,
+					 (int32) (state->ptr - state->begin));
 			}
 		}
 		else if (st == WVAL)
@@ -267,7 +273,9 @@ parse_hstore(HSParser *state)
 			}
 			else if (!isspace((unsigned char) *(state->ptr)))
 			{
-				elog(ERROR, "Syntax error near '%c' at position %d", *(state->ptr), (int32) (state->ptr - state->begin));
+				elog(ERROR, "Syntax error near \"%.*s\" at position %d",
+					 pg_mblen(state->ptr), state->ptr,
+					 (int32) (state->ptr - state->begin));
 			}
 		}
 		else
diff --git a/contrib/pageinspect/heapfuncs.c b/contrib/pageinspect/heapfuncs.c
index 11a910184b..f04455da12 100644
--- a/contrib/pageinspect/heapfuncs.c
+++ b/contrib/pageinspect/heapfuncs.c
@@ -30,6 +30,7 @@
 #include "catalog/pg_am_d.h"
 #include "catalog/pg_type.h"
 #include "funcapi.h"
+#include "mb/pg_wchar.h"
 #include "miscadmin.h"
 #include "pageinspect.h"
 #include "port/pg_bitutils.h"
@@ -99,7 +100,8 @@ text_to_bits(char *str, int len)
 		else
 			ereport(ERROR,
 					(errcode(ERRCODE_DATA_CORRUPTED),
-					 errmsg("illegal character '%c' in t_bits string", str[off])));
+					 errmsg("invalid character \"%.*s\" in t_bits string",
+							pg_mblen(str + off), str + off)));
 
 		if (off % 8 == 7)
 			bits[off / 8] = byte;
@@ -460,14 +462,13 @@ tuple_data_split(PG_FUNCTION_ARGS)
 		if (!t_bits_str)
 			ereport(ERROR,
 					(errcode(ERRCODE_DATA_CORRUPTED),
-					 errmsg("argument of t_bits is null, but it is expected to be null and %d character long",
-							bits_len)));
+					 errmsg("t_bits string must not be NULL")));
 
 		bits_str_len = strlen(t_bits_str);
 		if (bits_len != bits_str_len)
 			ereport(ERROR,
 					(errcode(ERRCODE_DATA_CORRUPTED),
-					 errmsg("unexpected length of t_bits %u, expected %d",
+					 errmsg("unexpected length of t_bits string: %u, expected %u",
 							bits_str_len, bits_len)));
 
 		/* do the conversion */
@@ -478,7 +479,7 @@ tuple_data_split(PG_FUNCTION_ARGS)
 		if (t_bits_str)
 			ereport(ERROR,
 					(errcode(ERRCODE_DATA_CORRUPTED),
-					 errmsg("t_bits string is expected to be NULL, but instead it is %zu bytes length",
+					 errmsg("t_bits string is expected to be NULL, but instead it is %zu bytes long",
 							strlen(t_bits_str))));
 	}
 
diff --git a/src/backend/utils/adt/encode.c b/src/backend/utils/adt/encode.c
index 61d318d93c..a609d49c12 100644
--- a/src/backend/utils/adt/encode.c
+++ b/src/backend/utils/adt/encode.c
@@ -15,6 +15,7 @@
 
 #include <ctype.h>
 
+#include "mb/pg_wchar.h"
 #include "utils/builtins.h"
 #include "utils/memutils.h"
 
@@ -171,17 +172,19 @@ hex_encode(const char *src, size_t len, char *dst)
 }
 
 static inline char
-get_hex(char c)
+get_hex(const char *cp)
 {
+	unsigned char c = (unsigned char) *cp;
 	int			res = -1;
 
-	if (c > 0 && c < 127)
-		res = hexlookup[(unsigned char) c];
+	if (c < 127)
+		res = hexlookup[c];
 
 	if (res < 0)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg("invalid hexadecimal digit: \"%c\"", c)));
+				 errmsg("invalid hexadecimal digit: \"%.*s\"",
+						pg_mblen(cp), cp)));
 
 	return (char) res;
 }
@@ -205,13 +208,15 @@ hex_decode(const char *src, size_t len, char *dst)
 			s++;
 			continue;
 		}
-		v1 = get_hex(*s++) << 4;
+		v1 = get_hex(s) << 4;
+		s++;
 		if (s >= srcend)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 					 errmsg("invalid hexadecimal data: odd number of digits")));
 
-		v2 = get_hex(*s++);
+		v2 = get_hex(s);
+		s++;
 		*p++ = v1 | v2;
 	}
 
@@ -338,7 +343,8 @@ pg_base64_decode(const char *src, size_t len, char *dst)
 			if (b < 0)
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-						 errmsg("invalid symbol \"%c\" while decoding base64 sequence", (int) c)));
+						 errmsg("invalid symbol \"%.*s\" found while decoding base64 sequence",
+								pg_mblen(s - 1), s - 1)));
 		}
 		/* add it to buffer */
 		buf = (buf << 6) + b;
diff --git a/src/backend/utils/adt/jsonpath_gram.y b/src/backend/utils/adt/jsonpath_gram.y
index f87db8ccf6..a5b461c567 100644
--- a/src/backend/utils/adt/jsonpath_gram.y
+++ b/src/backend/utils/adt/jsonpath_gram.y
@@ -526,8 +526,8 @@ makeItemLikeRegex(JsonPathParseItem *expr, JsonPathString *pattern,
 				ereport(ERROR,
 						(errcode(ERRCODE_SYNTAX_ERROR),
 						 errmsg("invalid input syntax for type %s", "jsonpath"),
-						 errdetail("unrecognized flag character \"%c\" in LIKE_REGEX predicate",
-								   flags->val[i])));
+						 errdetail("unrecognized flag character \"%.*s\" in LIKE_REGEX predicate",
+								   pg_mblen(flags->val+i), flags->val+i)));
 				break;
 		}
 	}
diff --git a/src/backend/utils/adt/regexp.c b/src/backend/utils/adt/regexp.c
index 06f808652a..c70c5eeeb3 100644
--- a/src/backend/utils/adt/regexp.c
+++ b/src/backend/utils/adt/regexp.c
@@ -423,8 +423,8 @@ parse_re_flags(pg_re_flags *flags, text *opts)
 				default:
 					ereport(ERROR,
 							(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-							 errmsg("invalid regular expression option: \"%c\"",
-									opt_p[i])));
+							 errmsg("invalid regular expression option: \"%.*s\"",
+									pg_mblen(opt_p + i), opt_p + i)));
 					break;
 			}
 		}
diff --git a/src/backend/utils/adt/varbit.c b/src/backend/utils/adt/varbit.c
index f0c6a44b84..3c03459f51 100644
--- a/src/backend/utils/adt/varbit.c
+++ b/src/backend/utils/adt/varbit.c
@@ -230,8 +230,8 @@ bit_in(PG_FUNCTION_ARGS)
 			else if (*sp != '0')
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
-						 errmsg("\"%c\" is not a valid binary digit",
-								*sp)));
+						 errmsg("\"%.*s\" is not a valid binary digit",
+								pg_mblen(sp), sp)));
 
 			x >>= 1;
 			if (x == 0)
@@ -255,8 +255,8 @@ bit_in(PG_FUNCTION_ARGS)
 			else
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
-						 errmsg("\"%c\" is not a valid hexadecimal digit",
-								*sp)));
+						 errmsg("\"%.*s\" is not a valid hexadecimal digit",
+								pg_mblen(sp), sp)));
 
 			if (bc)
 			{
@@ -531,8 +531,8 @@ varbit_in(PG_FUNCTION_ARGS)
 			else if (*sp != '0')
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
-						 errmsg("\"%c\" is not a valid binary digit",
-								*sp)));
+						 errmsg("\"%.*s\" is not a valid binary digit",
+								pg_mblen(sp), sp)));
 
 			x >>= 1;
 			if (x == 0)
@@ -556,8 +556,8 @@ varbit_in(PG_FUNCTION_ARGS)
 			else
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
-						 errmsg("\"%c\" is not a valid hexadecimal digit",
-								*sp)));
+						 errmsg("\"%.*s\" is not a valid hexadecimal digit",
+								pg_mblen(sp), sp)));
 
 			if (bc)
 			{
diff --git a/src/backend/utils/adt/varlena.c b/src/backend/utils/adt/varlena.c
index 2eaabd6231..df10bfb906 100644
--- a/src/backend/utils/adt/varlena.c
+++ b/src/backend/utils/adt/varlena.c
@@ -5586,8 +5586,8 @@ text_format(PG_FUNCTION_ARGS)
 		if (strchr("sIL", *cp) == NULL)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-					 errmsg("unrecognized format() type specifier \"%c\"",
-							*cp),
+					 errmsg("unrecognized format() type specifier \"%.*s\"",
+							pg_mblen(cp), cp),
 					 errhint("For a single \"%%\" use \"%%%%\".")));
 
 		/* If indirect width was specified, get its value */
@@ -5707,8 +5707,8 @@ text_format(PG_FUNCTION_ARGS)
 				/* should not get here, because of previous check */
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-						 errmsg("unrecognized format() type specifier \"%c\"",
-								*cp),
+						 errmsg("unrecognized format() type specifier \"%.*s\"",
+								pg_mblen(cp), cp),
 						 errhint("For a single \"%%\" use \"%%%%\".")));
 				break;
 		}

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