add missing requirements for internal file pointers

accepted/PSR-17-http-factory-meta.md

@@ -268,6 +268,34 @@ the time this proposal was developed allowed a string URI when creating a
 `UriInterface` implementation they provided. As such, accepting a string is
 expedient and follows existing semantics.
 
+### 5.7 Why are Streams created with the read/write pointer at the beginning?
+
+The primary use case of `StreamFactoryInterface::createStream()` and
+`StreamFactoryInterface::createStreamFromFile()` is to create streams for the
+body of a `ResponseInterface` instance to be emitted as an HTTP response.
+
+An emitter can make no assumptions about streams (or their internal resource
+handles) being repeatable, and therefore can't simply `rewind()` the stream.
+
+Even in the case where a stream returns `true` for `isSeekable()`, rewinding
+the stream could have unacceptable performance implications. Similarly, the
+`__toString()` method results in unacceptable memory overhead for large streams.
+
+Consequently, to efficiently emit the body stream of a `ResponseInterface`
+instance as body of an HTTP response, we must be able to make an assumption
+about the stream pointer being positioned at the beginning of the stream.
+
+As for `StreamFactoryInterface::createStreamFromResource()`, the caller is
+responsible for the creation of the PHP resource to use as the basis for the
+stream, and therefore also assumes responsibility for the read/write pointer
+state of the resource and resulting stream.
+
+And finally, the instances created by the `RequestFactoryInterface()`,
+`ResponseFactoryInterface()` and `ServerRequestFactoryInterface` factories
+must generate instances that return an empty body-stream in read and write
+mode, so that client-code can start writing to the body stream without
+manually needing to initialize anything else.
+
 ## 6. People
 
 This PSR was produced by a FIG Working Group with the following members:
@@ -306,3 +334,23 @@ _**Note:** Order descending chronologically._
 - [shadowhand Dependency Inversion and PSR-7 Bodies](http://shadowhand.me/dependency-inversion-and-psr-7-bodies/)
 - [PHP-FIG mailing list thread discussing factories](https://groups.google.com/d/msg/php-fig/G5pgQfQ9fpA/UWeM1gm1CwAJ)
 - [PHP-FIG mailing list thread feedback on proposal](https://groups.google.com/d/msg/php-fig/piRtB2Z-AZs/8UIwY1RtDgAJ)
+
+## 9. Errata
+
+This recommendation initially omitted the following requirements.
+
+### 9.1 `StreamFactoryInterface::createStream()`
+
+The state of the created temporary resource was unspecified - per section 5.7, the
+requirement was added to position the temporary resource at the beginning of the stream.
+
+### 9.2 `StreamFactoryInterface::createStreamFromFile()`
+
+The state of the created temporary resource was unspecified - per section 5.7, the
+requirement was added to position the temporary resource at the beginning of the stream.
+
+### 9.3 `StreamFactoryInterface::createStreamFromResource()`
+
+The requirements did not specify whether this method might affect the state of the given
+resource - per section 5.7, the requirement was added to clarify that this method MUST NOT
+modify the position of the given resource.

accepted/PSR-17-http-factory.md

@@ -51,6 +51,8 @@ interface RequestFactoryInterface
 }
 ```
 
+The created instance MUST return an empty body-stream in read and write mode.
+
 ### 2.2 ResponseFactoryInterface
 
 Has the ability to create responses.
@@ -74,6 +76,8 @@ interface ResponseFactoryInterface
 }
 ```
 
+The created instance MUST return an empty body-stream in read and write mode.
+
 ### 2.3 ServerRequestFactoryInterface
 
 Has the ability to create server requests.
@@ -102,6 +106,8 @@ interface ServerRequestFactoryInterface
 }
 ```
 
+The created instance MUST return an empty body-stream in read and write mode.
+
 ### 2.4 StreamFactoryInterface
 
 Has the ability to create streams for requests and responses.
@@ -117,6 +123,9 @@ interface StreamFactoryInterface
      * Create a new stream from a string.
      *
      * The stream SHOULD be created with a temporary resource.
+     * 
+     * The stream MUST be created with the current position of the file read/write
+     * pointer at the beginning of the stream. (Errata 9.1)
      *
      * @param string $content String content with which to populate the stream.
      */
@@ -130,6 +139,9 @@ interface StreamFactoryInterface
      *
      * The `$filename` MAY be any string supported by `fopen()`.
      *
+     * The stream MUST be created with the current position of the file read/write
+     * pointer at the beginning of the stream. (Errata 9.2)
+     *
      * @param string $filename The filename or stream URI to use as basis of stream.
      * @param string $mode The mode with which to open the underlying filename/stream.
      *
@@ -143,6 +155,8 @@ interface StreamFactoryInterface
      *
      * The stream MUST be readable and may be writable.
      *
+     * The current read/write position of the given PHP resource MUST NOT be modified. (Errata 9.3)
+     *
      * @param resource $resource The PHP resource to use as the basis for the stream.
      */
     public function createStreamFromResource($resource): StreamInterface;