[go] mime: reject duplicate formatted parameter names

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shuang cui (Gerrit)

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Jun 13, 2026, 9:41:18โ€ฏAMJun 13
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shuang cui has uploaded the change for review

Commit message

mime: reject duplicate formatted parameter names

FormatMediaType lower-cases parameter names when serializing.
If the input map contains keys that differ only by case, it
can emit duplicate parameter names such as "name=foo; name=bar",
which ParseMediaType rejects. Return an empty string in this case,
matching FormatMediaType's documented behavior for invalid output.
Change-Id: I6d0890bf608da8c3a18af60a36b1825f8dc88ce9

Change diff

diff --git a/src/mime/mediatype.go b/src/mime/mediatype.go
index c6006b6..520bcb7 100644
--- a/src/mime/mediatype.go
+++ b/src/mime/mediatype.go
@@ -34,6 +34,7 @@
 		b.WriteString(strings.ToLower(sub))
 	}
 
+	seenAttrs := make(map[string]struct{}, len(param))
 	for _, attribute := range slices.Sorted(maps.Keys(param)) {
 		value := param[attribute]
 		b.WriteByte(';')
@@ -41,7 +42,12 @@
 		if !isToken(attribute) {
 			return ""
 		}
-		b.WriteString(strings.ToLower(attribute))
+		attribute = strings.ToLower(attribute)
+		if _, ok := seenAttrs[attribute]; ok {
+			return ""
+		}
+		seenAttrs[attribute] = struct{}{}
+		b.WriteString(attribute)
 
 		needEnc := needsEncoding(value)
 		if needEnc {
diff --git a/src/mime/mediatype_test.go b/src/mime/mediatype_test.go
index da8d64d..343beb8 100644
--- a/src/mime/mediatype_test.go
+++ b/src/mime/mediatype_test.go
@@ -531,6 +531,7 @@
 	{"foo/BAR", map[string]string{"both": `With \backslash and "quote`}, `foo/bar; both="With \\backslash and \"quote"`},
 	{"foo/BAR", map[string]string{"": "empty attribute"}, ""},
 	{"foo/BAR", map[string]string{"bad attribute": "baz"}, ""},
+	{"foo/BAR", map[string]string{"Name": "foo", "name": "bar"}, ""},
 	{"foo/BAR", map[string]string{"nonascii": "not an ascii character: รค"}, "foo/bar; nonascii*=utf-8''not%20an%20ascii%20character%3A%20%C3%A4"},
 	{"foo/BAR", map[string]string{"ctl": "newline: \n nil: \000"}, "foo/bar; ctl*=utf-8''newline%3A%20%0A%20nil%3A%20%00"},
 	{"foo/bar", map[string]string{"a": "av", "b": "bv", "c": "cv"}, "foo/bar; a=av; b=bv; c=cv"},

Change information

Files:
  • M src/mime/mediatype.go
  • M src/mime/mediatype_test.go
Change size: XS
Delta: 2 files changed, 8 insertions(+), 1 deletion(-)
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shuang cui (Gerrit)

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Jun 13, 2026, 9:43:17โ€ฏAMJun 13
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shuang cui voted Commit-Queue+1

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shuang cui (Gerrit)

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Jun 15, 2026, 8:43:57โ€ฏAMJun 15
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shuang cui . resolved

Hi, Would you mind review this PR at your convenience? Thank you very much.

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shuang cui (Gerrit)

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