Can Marshalling a Go `map[string]string` to JSON Fail, and Under What Circumstances?

Can Marshalling a Map[string]string to JSON Return an Error?

In general, marshalling a valid map[string]string to JSON using json.Marshal() is unlikely to result in an error. This is because both keys and values in JSON are valid Unicode strings which Go represents using UTF-8 encoded byte sequences.

However, there are a few exceptional situations to consider:

• Invalid UTF-8: Mashalling a string with invalid UTF-8 characters will not cause an error, but the characters will be replaced with the Unicode replacement character (U FFFD).

m := map[string]string{"\xff": "a"}
data, err := json.Marshal(m)
// Output: {"\ufffd":"a"} <nil></nil>

• Concurrent Map Modification: Running json.Marshal() on a map that is being concurrently modified by another goroutine can trigger the runtime's concurrent map misuse detection. This will result in a fatal error.

m := map[string]string{"a": "b"}

go func() {
    for {
        m["c"] = "d"
    }
}()

for {
    if _, err := json.Marshal(m); err != nil {
        // Error: "concurrent map iteration and map write"
    }
}

• Out of Memory: Extreme cases of memory exhaustion may terminate the application before json.Marshal() returns.

While it is generally not necessary to handle errors when marshalling map[string]string to JSON, good programming practices dictate checking for errors in all cases, even in situations where the likelihood of an error is low.

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