Add path selector context and explicit path query

[lschulz]

Adds a context pointer to the path selector's Path() method that allows applications to pass arbitrary context data to their custom path selector. Currently Path() has to select a path without knowing anything about the packet it picks a path for. The context can provide additional information, e.g., whether the packet is urgent or not.

Adds an exported function for querying paths to a particular destination AS. This function was missing before, making it very hard to send a packet to a new destination on a ListenConn.

Adds a getter for the size of a path in the dataplane. The path size can be used with the MTU metadata to approximate the maximum payload size per packet.

Other changes:

• Return an error if SCION host context initialization fails (e.g., because one forgot to set SCION_DAEMON_ADDRESS). Previously PAN terminated the application in this case even though a connection error is very likely recoverable. Returning an error to the application is especially important for GUI apps that do not have a terminal PAN could print anything to.

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This change is 

[JordiSubira]

Reviewed 10 of 10 files at r1, all commit messages.
Reviewable status: all files reviewed, 11 unresolved discussions / 0 of 1 LGTMs obtained / 0 of 1 approvals obtained

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pkg/pan/udp_listen.go line 57 at r1 (raw file):

	ReadFromVia(b []byte) (int, UDPAddr, *Path, error)
	// WriteToWithCtx writes a message to the remote address using a path from
	// the path selector. ctx is passed to the path selector.

Similar comment to the counterpart function in udp_dial.go

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pkg/pan/udp_listen.go line 180 at r1 (raw file):

}

func (s *DefaultReplySelector) Path(ctx interface{}, remote UDPAddr) *Path {

_ (unused parameter)

Code quote:

ctx

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pkg/pan/sciond.go line 52 at r1 (raw file):

func (e HostContextError) Error() string {
	return fmt.Sprintf("error initializing SCION host context: '%s'", e.Cause)

%v

Code quote:

%s

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pkg/pan/path.go line 47 at r1 (raw file):

}

// DataplaneLen returns the legnth of the path in the data plane.

length

Code quote:

legnth

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pkg/pan/path.go line 48 at r1 (raw file):

// DataplaneLen returns the legnth of the path in the data plane.
func (p *Path) DataplaneLen() (int, error) {

Currently this function isn't used anywhere, could we add some small UT?

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ssh/client/ssh/selector.go line 58 at r1 (raw file):

}

func (s *roundRobinSelector) Path(cts interface{}) *pan.Path {

_ (unused parameter)

Code quote:

cts

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ssh/client/ssh/selector.go line 97 at r1 (raw file):

}

func (s *randomSelector) Path(ctx interface{}) *pan.Path {

_ (unused parameter)

Code quote:

ctx

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pkg/pan/selector.go line 34 at r1 (raw file):

	// Path selects the path for the next packet.
	// Invoked for each packet sent with Write.
	Path(ctx interface{}) *Path

Is there a reason for not making this "context.Context"

Code quote:

interface{}

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pkg/pan/selector.go line 67 at r1 (raw file):

}

func (s *DefaultSelector) Path(ctx interface{}) *Path {

_ (unused parameter)

Code quote:

ctx

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pkg/pan/selector.go line 146 at r1 (raw file):

}

func (s *PingingSelector) Path(ctx interface{}) *Path {

_ (unused parameter)

Code quote:

ctx

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pkg/pan/udp_dial.go line 32 at r1 (raw file):

	SetPolicy(policy Policy)
	// WriteWithCtx writes a message to the remote address using a path from the
	// path policy and selector. ctx is passed to the path selector.

It may be useful to extend this comment with the brief explanation you have added to this PR,i.e.,: "he context can provide additional information, e.g., whether the packet is urgent or not.", or a similar one.

[lschulz]

Reviewable status: 4 of 11 files reviewed, 11 unresolved discussions / 0 of 1 LGTMs obtained / 0 of 1 approvals obtained

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pkg/pan/path.go line 47 at r1 (raw file):

Previously, JordiSubira wrote…

length

Done.

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pkg/pan/path.go line 48 at r1 (raw file):

Previously, JordiSubira wrote…

Currently this function isn't used anywhere, could we add some small UT?

Done.

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pkg/pan/sciond.go line 52 at r1 (raw file):

Previously, JordiSubira wrote…

%v

Done.

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pkg/pan/selector.go line 34 at r1 (raw file):

Previously, JordiSubira wrote…

Is there a reason for not making this "context.Context"

I thought about making it a context.Context, but I didn't see a reason to restrict user code to a specific type or interface. That being said, I'm not against using context.Context if this is more idiomatic, or if PAN would need to interact with this context in the future.

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pkg/pan/selector.go line 67 at r1 (raw file):

Previously, JordiSubira wrote…

_ (unused parameter)

Done.

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pkg/pan/selector.go line 146 at r1 (raw file):

Previously, JordiSubira wrote…

_ (unused parameter)

Done.

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pkg/pan/udp_dial.go line 32 at r1 (raw file):

Previously, JordiSubira wrote…

It may be useful to extend this comment with the brief explanation you have added to this PR,i.e.,: "he context can provide additional information, e.g., whether the packet is urgent or not.", or a similar one.

Done.

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pkg/pan/udp_listen.go line 57 at r1 (raw file):

Previously, JordiSubira wrote…

Similar comment to the counterpart function in udp_dial.go

Done.

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pkg/pan/udp_listen.go line 180 at r1 (raw file):

Previously, JordiSubira wrote…

_ (unused parameter)

Done.

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ssh/client/ssh/selector.go line 58 at r1 (raw file):

Previously, JordiSubira wrote…

_ (unused parameter)

Done.

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ssh/client/ssh/selector.go line 97 at r1 (raw file):

Previously, JordiSubira wrote…

_ (unused parameter)

Done.

[JordiSubira]

Reviewed 3 of 7 files at r2, all commit messages.
Reviewable status: 7 of 11 files reviewed, 1 unresolved discussion / 0 of 1 LGTMs obtained / 0 of 1 approvals obtained

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pkg/pan/selector.go line 34 at r1 (raw file):

Previously, lschulz (Lars-Christian Schulz) wrote…

I thought about making it a context.Context, but I didn't see a reason to restrict user code to a specific type or interface. That being said, I'm not against using context.Context if this is more idiomatic, or if PAN would need to interact with this context in the future.

I do believe it is more idiomatic and more consistent with the existing code which uses context.Context, so if there's no strong reason, I'd opt for using it here as well.

[lschulz]

Reviewable status: 5 of 11 files reviewed, 1 unresolved discussion / 0 of 1 LGTMs obtained / 0 of 1 approvals obtained

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pkg/pan/selector.go line 34 at r1 (raw file):

Previously, JordiSubira wrote…

I do believe it is more idiomatic and more consistent with the existing code which uses context.Context, so if there's no strong reason, I'd opt for using it here as well.

Ok, I switched to context.Context.

[JordiSubira]

Reviewed 6 of 6 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved