panic: nil map in alertmanager email.New() — EmailConfig.Headers not initialized

[justinb-dfw]

Bug Report

Root Cause: notify/email/email.go, function New() (line ~54-58), accesses the c.Headers map without first checking if it is nil. When an EmailConfig is created programmatically by the query-service pushing channel config via /api/v1/routes without an explicit headers: YAML field, Headers is a nil map[string]string. Map reads succeed (returning zero value + false), but the subsequent map writes panic.

Stack trace:

http: panic serving: assignment to entry in nil map
github.com/prometheus/alertmanager/notify/email.New(...)
        /app/notify/email/email.go:55

Buggy code in notify/email/email.go lines 54-65:

func New(c *config.EmailConfig, t *template.Template, l log.Logger) *Email {
    if _, ok := c.Headers["Subject"]; !ok {    // panics if c.Headers is nil
        c.Headers["Subject"] = config.DefaultEmailSubject
    }
    if _, ok := c.Headers["To"]; !ok {
        c.Headers["To"] = c.To
    }
    if _, ok := c.Headers["From"]; !ok {
        c.Headers["From"] = c.From
    }
    ...
}

Fix — one-line nil guard before first map access:

func New(c *config.EmailConfig, t *template.Template, l log.Logger) *Email {
    if c.Headers == nil {
        c.Headers = make(map[string]string)
    }
    if _, ok := c.Headers["Subject"]; !ok {
        c.Headers["Subject"] = config.DefaultEmailSubject
    }
    ...
}

Reproduction:

1. Deploy SigNoz with alertmanager signoz/alertmanager:latest (v0.23.9)
2. Create an email notification channel via SigNoz UI (Settings → Alert Channels)
3. Click Test — query-service returns 500, alertmanager pod logs show the panic above

Workaround until fixed: Pre-configure the alertmanager alertmanager.yml with a default receiver containing an explicit headers: block so the Headers map is initialized from YAML deserialization rather than being nil.

Environment: signoz/alertmanager:latest (v0.23.9), linux/arm64 and amd64, triggered via /api/v1/testReceiver endpoint.

[welcome]

Thanks for opening this issue. A team member should give feedback soon. In the meantime, feel free to check out the contributing guidelines.

[onthebed]

Looking into this — I’ll trace the alertmanager email config path and try to pin down where headers should be initialized before email.New() runs.

[onthebed]

Looking into this — I’m checking the alertmanager email config path to guard nil headers before they’re mutated. Happy to submit a fix.