notify: add mechanisms for email message forwarding

As preparation for the integration of `proxmox-mail-foward` into the
notification system, this commit makes a few changes that allow us to
forward raw email messages (as passed from postfix).

For mail-based notification targets, the email will be forwarded
as-is, including all headers. The only thing that changes is the
message envelope.
For other notification targets, the mail is parsed using the
`mail-parser` crate, which allows us to extract a subject and a body.
As a body we use the plain-text version of the mail. If an email is
HTML-only, the `mail-parser` crate will automatically attempt to
transform the HTML into readable plain text.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>

Cargo.toml

@@ -64,6 +64,7 @@ lazy_static = "1.4"
 ldap3 = { version = "0.11", default-features = false }
 libc = "0.2.107"
 log = "0.4.17"
+mail-parser = "0.8.2"
 native-tls = "0.2"
 nix = "0.26.1"
 once_cell = "1.3.1"

proxmox-notify/Cargo.toml

@@ -12,6 +12,7 @@ anyhow.workspace = true
 handlebars = { workspace = true }
 lazy_static.workspace = true
 log.workspace = true
+mail-parser = { workspace = true, optional = true }
 once_cell.workspace = true
 openssl.workspace = true
 proxmox-http = { workspace = true, features = ["client-sync"], optional = true }
@@ -28,5 +29,6 @@ serde_json.workspace = true
 
 [features]
 default = ["sendmail", "gotify"]
+mail-forwarder = ["dep:mail-parser"]
 sendmail = ["dep:proxmox-sys"]
 gotify = ["dep:proxmox-http"]

proxmox-notify/src/endpoints/gotify.rs

@@ -19,6 +19,7 @@ fn severity_to_priority(level: Severity) -> u32 {
         Severity::Notice => 3,
         Severity::Warning => 5,
         Severity::Error => 9,
+        Severity::Unknown => 3,
     }
 }
 
@@ -94,6 +95,8 @@ impl Endpoint for GotifyEndpoint {
 
                 (rendered_title, rendered_message)
             }
+            #[cfg(feature = "mail-forwarder")]
+            Content::ForwardedMail { title, body, .. } => (title.clone(), body.clone()),
         };
 
         // We don't have a TemplateRenderer::Markdown yet, so simply put everything

proxmox-notify/src/endpoints/sendmail.rs

@@ -134,6 +134,11 @@ impl Endpoint for SendmailEndpoint {
                 )
                 .map_err(|err| Error::NotifyFailed(self.config.name.clone(), err.into()))
             }
+            #[cfg(feature = "mail-forwarder")]
+            Content::ForwardedMail { raw, uid, .. } => {
+                proxmox_sys::email::forward(&recipients_str, &mailfrom, raw, *uid)
+                    .map_err(|err| Error::NotifyFailed(self.config.name.clone(), err.into()))
+            }
         }
     }
 

proxmox-notify/src/lib.rs

@@ -102,6 +102,8 @@ pub enum Severity {
     Warning,
     /// Error
     Error,
+    /// Unknown severity (e.g. forwarded system mails)
+    Unknown,
 }
 
 impl Display for Severity {
@@ -111,6 +113,7 @@ impl Display for Severity {
             Severity::Notice => f.write_str("notice"),
             Severity::Warning => f.write_str("warning"),
             Severity::Error => f.write_str("error"),
+            Severity::Unknown => f.write_str("unknown"),
         }
     }
 }
@@ -123,6 +126,7 @@ impl FromStr for Severity {
             "notice" => Ok(Self::Notice),
             "warning" => Ok(Self::Warning),
             "error" => Ok(Self::Error),
+            "unknown" => Ok(Self::Unknown),
             _ => Err(Error::Generic(format!("invalid severity {s}"))),
         }
     }
@@ -148,6 +152,18 @@ pub enum Content {
         /// Data that can be used for template rendering.
         data: Value,
     },
+    #[cfg(feature = "mail-forwarder")]
+    ForwardedMail {
+        /// Raw mail contents
+        raw: Vec<u8>,
+        /// Fallback title
+        title: String,
+        /// Fallback body
+        body: String,
+        /// UID to use when calling sendmail
+        #[allow(dead_code)] // Unused in some feature flag permutations
+        uid: Option<u32>,
+    },
 }
 
 #[derive(Debug, Clone)]
@@ -190,6 +206,31 @@ impl Notification {
             },
         }
     }
+    #[cfg(feature = "mail-forwarder")]
+    pub fn new_forwarded_mail(raw_mail: &[u8], uid: Option<u32>) -> Result<Self, Error> {
+        let message = mail_parser::Message::parse(raw_mail)
+            .ok_or_else(|| Error::Generic("could not parse forwarded email".to_string()))?;
+
+        let title = message.subject().unwrap_or_default().into();
+        let body = message.body_text(0).unwrap_or_default().into();
+
+        Ok(Self {
+            // Unfortunately we cannot reasonably infer the severity from the
+            // mail contents, so just set it to the highest for now so that
+            // it is not filtered out.
+            content: Content::ForwardedMail {
+                raw: raw_mail.into(),
+                title,
+                body,
+                uid,
+            },
+            metadata: Metadata {
+                severity: Severity::Unknown,
+                additional_fields: Default::default(),
+                timestamp: proxmox_time::epoch_i64(),
+            },
+        })
+    }
 }
 
 /// Notification configuration