Modify Mail to allow folding of long header fields

Sources/SwiftSMTP/Mail.swift

@@ -15,6 +15,7 @@
  **/
 
 import Foundation
+import LoggerAPI
 
 /// Represents an email that can be sent through an `SMTP` instance.
 public struct Mail {
@@ -122,27 +123,78 @@ public struct Mail {
         dictionary["MESSAGE-ID"] = id
         dictionary["DATE"] = Date().smtpFormatted
         dictionary["FROM"] = from.mime
-        dictionary["TO"] = to.map { $0.mime }.joined(separator: ", ")
+        dictionary["TO"] = foldHeaderValue(key: "TO", value: to.map { $0.mime }.joined(separator: ", "))
 
         if !cc.isEmpty {
-            dictionary["CC"] = cc.map { $0.mime }.joined(separator: ", ")
+            dictionary["CC"] = foldHeaderValue(key: "CC", value: cc.map { $0.mime }.joined(separator: ", "))
         }
 
-        dictionary["SUBJECT"] = subject.mimeEncoded ?? ""
+        dictionary["SUBJECT"] = foldHeaderValue(key: "SUBJECT", value: (subject.mimeEncoded ?? ""))
         dictionary["MIME-VERSION"] = "1.0 (Swift-SMTP)"
 
         for (key, value) in additionalHeaders {
             let keyUppercased = key.uppercased()
             if  keyUppercased != "CONTENT-TYPE" &&
                 keyUppercased != "CONTENT-DISPOSITION" &&
                 keyUppercased != "CONTENT-TRANSFER-ENCODING" {
-                dictionary[keyUppercased] = value
+                dictionary[keyUppercased] = foldHeaderValue(key: key, value: value)
             }
         }
 
         return dictionary
     }
 
+    private func foldHeaderValue(key: String, value: String) -> String {
+        let suggestedLineLength = 78
+        let maximumLineLength = 998
+
+        let initialHeader = "\(key): \(value)"
+        if initialHeader.count <= suggestedLineLength {
+            return value
+        }
+        // if we're here, it means that RFC 5322 SUGGESTS that we fold this header
+        var foldedHeader = ""
+        var register = "\(key): "
+        var linePosition = 0
+        let foldableCharacters = CharacterSet(charactersIn: " ,")
+        for char in value {
+            // append the character to the register
+            register.append(char)
+            // this test is to detect the end of a token, mid-stream
+            if let _ = String(char).rangeOfCharacter(from: foldableCharacters) {
+                if linePosition > 1 && (register.count + linePosition > suggestedLineLength) {
+                    // We already have stuff on the line and the register is too long
+                    // to continue on the current line. So we fold and start a new line.
+                    foldedHeader.append("\r\n ")
+                    linePosition = 1
+                }
+                // now, register contains a complete token, so we put it up to the line
+                linePosition += register.count
+                if linePosition > maximumLineLength {
+                    Log.error("Header line length exceeds the specified maximum (998 chars) - see RFC 5322 section 2.2.3")
+                }
+                foldedHeader.append(register)
+                register = ""
+            }
+        }
+        // We have the last of the value characters in register, so we put them up
+        // to the line. We still want to apply the same logic as that inside the loop
+        // though, and apply folding if it's appropriate.
+        if linePosition > 1 && (register.count + linePosition > suggestedLineLength) {
+            foldedHeader.append("\r\n ")
+            linePosition = 1
+        }
+        if (register.count + linePosition) > maximumLineLength {
+            Log.error("Header line length exceeds the specified maximum (998 chars) - see RFC 5322 section 2.2.3")
+        }
+        foldedHeader.append(register)
+
+        // Here is where we remove "\(key): " from the beginning of the folded header,
+        // so we only return the value.
+        let valueIndex = foldedHeader.index(foldedHeader.startIndex, offsetBy: key.count + 2)
+        return String(foldedHeader.suffix(from: valueIndex))
+    }
+
     var headersString: String {
         return headersDictionary.map { (key, value) in
             return "\(key): \(value)"

Tests/SwiftSMTPTests/TestHeaderFolding.swift

@@ -0,0 +1,138 @@
+/**
+ * Copyright Kitura 2021-2022
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ **/
+
+@testable import SwiftSMTP
+import LoggerAPI
+import XCTest
+
+internal class TestLogger: Logger {
+    var messages = [String]()
+
+    func log(_ type: LoggerMessageType, msg: String, functionName: String, lineNum: Int, fileName: String) {
+        let message = "[\(type.description.uppercased())] \(msg)"
+        messages.append(message)
+    }
+
+    func isLogging(_ level: LoggerMessageType) -> Bool { true }
+}
+
+class TestHeaderFolding: XCTestCase {
+    let spaciousSubject = "Sample subject  with  extra whitespace inside"
+    var mailMessage: Mail {
+        Mail(from: Mail.User(name: "Test User", email: "[email protected]"),
+                           to: manyRecipients,
+                           cc: [absurdlyLongEmail],
+                           subject: spaciousSubject,
+                           text: "Just some message",
+                           additionalHeaders: ["X-OBNOXIOUS": egregiousLine,
+                                               "X-SHORT": spaciousSubject])
+    }
+
+    let manyRecipients: [Mail.User] = Array.init(repeating: Mail.User(email: "[email protected]"),
+                                                 count: 30)
+    let absurdlyLongEmail = Mail.User(name: "Unfortunate Joe",
+                                      email: "unfortunate_joe_1234567890_123456789_123456783_123456784_123456785_123456786@dumbster.local")
+
+    // this is a base64 encoding of Package.swift
+    let egregiousLine = "Ly8gc3dpZnQtdG9vbHMtdmVyc2lvbjo1LjAKCmltcG9ydCBQYWNrYWdlRGVzY3JpcHRpb24KCmxldCBwYWNrYWdlID0gUGFja2FnZSgKICAgIG5hbWU6ICJTd2lmdFNNVFAiLAogICAgcHJvZHVjdHM6IFsKICAgICAgICAubGlicmFyeSgKICAgICAgICAgICAgbmFtZTogIlN3aWZ0U01UUCIsCiAgICAgICAgICAgIHRhcmdldHM6IFsiU3dpZnRTTVRQIl0pLAogICAgICAgIF0sCiAgICBkZXBlbmRlbmNpZXM6IFsKICAgICAgICAucGFja2FnZSh1cmw6ICJodHRwczovL2dpdGh1Yi5jb20vS2l0dXJhL0JsdWVTb2NrZXQuZ2l0IiwgZnJvbTogIjIuMC4yIiksCiAgICAgICAgLnBhY2thZ2UodXJsOiAiaHR0cHM6Ly9naXRodWIuY29tL0tpdHVyYS9CbHVlU1NMU2VydmljZS5naXQiLCBmcm9tOiAiMi4wLjEiKSwKICAgICAgICAucGFja2FnZSh1cmw6ICJodHRwczovL2dpdGh1Yi5jb20vS2l0dXJhL0JsdWVDcnlwdG9yLmdpdCIsIGZyb206ICIyLjAuMSIpLAogICAgICAgIC5wYWNrYWdlKHVybDogImh0dHBzOi8vZ2l0aHViLmNvbS9LaXR1cmEvTG9nZ2VyQVBJLmdpdCIsIGZyb206ICIxLjkuMjAwIiksCiAgICAgICAgXSwKICAgIHRhcmdldHM6IFsKICAgICAgICAudGFyZ2V0KAogICAgICAgICAgICBuYW1lOiAiU3dpZnRTTVRQIiwKICAgICAgICAgICAgZGVwZW5kZW5jaWVzOiBbIlNvY2tldCIsICJTU0xTZXJ2aWNlIiwgIkNyeXB0b3IiLCAiTG9nZ2VyQVBJIl0pLAogICAgICAgIC50ZXN0VGFyZ2V0KAogICAgICAgICAgICBuYW1lOiAiU3dpZnRTTVRQVGVzdHMiLAogICAgICAgICAgICBkZXBlbmRlbmNpZXM6IFsiU3dpZnRTTVRQIl0pLAogICAgICAgIF0KKQo="
+
+    var previousLogger: Logger?
+    var currentLogger: TestLogger?
+
+    override func setUpWithError() throws {
+        previousLogger = Log.logger
+        currentLogger = TestLogger()
+        Log.logger = currentLogger
+    }
+
+    override func tearDownWithError() throws {
+        Log.logger = previousLogger
+        currentLogger = nil
+    }
+
+    // Here we test two things: 1) a short line will not be folded,
+    // and 2) a line containing multiple consecutive whitespace characters
+    // will not be changed.
+    func testShortHeaderUnchanged() throws {
+        let allHeaders = mailMessage.headersString.split(separator: "\r\n")
+        let unmodified = "X-SHORT: \(spaciousSubject)"
+        for header in allHeaders where header.hasPrefix("X-SHORT") {
+            XCTAssertEqual(String(header), unmodified)
+        }
+    }
+
+    // Whatever else is true of the folded header, the folding process should
+    // not ever produce a line which consists solely of whitespace.
+    func testNoWSOnlyLines() throws {
+        let allHeaders = mailMessage.headersString.split(separator: "\r\n")
+        for header in allHeaders {
+            XCTAssert(!header.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty)
+        }
+    }
+
+    // This test looks at folding a long name plus email address, which won't
+    // contain a comma. The fold should happen at the space between the name
+    // and the email.
+    func testAbsurdEmail() throws {
+        let allHeaders = mailMessage.headersString.split(separator: "\r\n")
+        var foundCC = false
+        var testEncountered = false
+        for header in allHeaders {
+            if foundCC {
+                // we are now at the first line *after* the CC: line
+                XCTAssertEqual(header, " <unfortunate_joe_1234567890_123456789_123456783_123456784_123456785_123456786@dumbster.local>")
+                testEncountered = true
+                break
+            } else if header.hasPrefix("CC:") {
+                foundCC = true
+                XCTAssertEqual(header, "CC: \("Unfortunate Joe".mimeEncoded!) ")
+            }
+        }
+        XCTAssertTrue(testEncountered, "The CC line did not get folded!")
+    }
+
+    // This test looks at what happens when a header value does not
+    // contain any spaces at all, and is thus unfoldable, while still
+    // being longer than the recommended length.
+    func testUnfoldableHeader() throws {
+        let allHeaders = mailMessage.headersString.split(separator: "\r\n")
+        for header in allHeaders where header.hasPrefix("X-OBNOXIOUS") {
+            XCTAssertEqual(header, "X-OBNOXIOUS: \(egregiousLine)")
+        }
+        // Incidentally, the line is longer than the RFC mandated maximum length.
+        // Currently, the library does not throw an error in this case, but
+        // the folding code should at least log a message about it, so that
+        // consumers of the library can have some help figuring out why their
+        // mail message was rejected by a remote SMTP server.
+        guard let currentLogger = currentLogger else {
+            throw XCTSkip("There is no logger installed!")
+        }
+        XCTAssert(!currentLogger.messages
+                    .filter({ $0.hasPrefix("[ERROR] Header line length") })
+                    .isEmpty)
+    }
+
+    // This test looks at a long list of email addresses. There are plenty
+    // of commas and whitespaces, so folding is possible. It should happen
+    // at the whitespace, and not in the middle of an address.
+    func testFoldOnWhitespace() throws {
+        let allHeaders = mailMessage
+            .headersString.split(separator: "\r\n")
+        for header in allHeaders where header.hasPrefix("TO: ") {
+            XCTAssert(header.hasSuffix("[email protected], "))
+        }
+    }
+}